find_package does not find Boost

Our project is building and running on Windows 7 and Ubuntu 16.04. I am now trying to cross compile it under licensed code, copying our project under the external directory. I added BOOST_ROOT, BOOST_INCLUDEDIR and Boost_DIR to the terminal but they get stripped from the environment.
I have:
cmake version 3.15.3

Boost is in:
/usr/local/lib/cmake/Boost-1.71.0/BoostConfig.cmake
/usr/local/lib/cmake/boost_XX-1.71.0 folders
/usr/local/lib/libboost_XX.so.1.71.0 files

The top of my CMakeLists.txt file, including diagnostics:
cmake_minimum_required(VERSION 3.12)

project(MyProject CXX)
message("Running CMakeLists for MyProject!")
# Options
option(GEN_DOCS "Generate documentation" OFF)
option(BUILD_TESTS "Build test executable" ON)
option(INSTALL_LIBRARY "Install the library" OFF)
message("(env)BOOST_ROOT is $ENV{BOOST_ROOT}") #2020-01-14
message("(env)Boost_DIR is $ENV{Boost_DIR}") #2020-01-15
if((${CMAKE_SYSTEM_NAME} STREQUAL "Linux") AND (NOT DEFINED BOOST_ROOT))
    message(STATUS "setting boost_root for Linux")
#    SET(BOOST_ROOT "/usr/local/lib/cmake/Boost-1.71.0")
#    SET(BOOST_ROOT "/usr/local/lib/cmake")
#    SET(BOOST_ROOT "/usr/local/lib")
    SET(BOOST_ROOT "/usr/local")
endif()
message("BOOST_ROOT is " ${BOOST_ROOT}) #2020-01-15
find_package(Boost 1.71.0 REQUIRED COMPONENTS system date_time filesystem)
message("Boost_FOUND is " ${Boost_FOUND}) #2020-01-15
message("Boost_INCLUDE_DIR is " ${Boost_INCLUDE_DIR}) #2020-01-15
message("Boost_DIR is " ${Boost_DIR}) #2020-01-15
message("Boost_CONFIG is " ${Boost_CONFIG}) #2020-01-15
message("Boost_VERSION is " ${Boost_VERSION}) #2020-01-15

The output includes:
| Running CMakeLists for MyProject!
| (env)BOOST_ROOT is
| (env)Boost_DIR is
| – setting boost_root for Linux
| BOOST_ROOT is /usr/local
| CMake Warning at /home/me/dev/source/github/MyProject-build-yocto/—/usr/share/cmake-3.12/Modules/FindBoost.cmake:577 (message):
| Imported targets and dependency information not available for Boost version
| (all versions older than 1.33)

and farther down
| CMake Error at /home/paradigm/dev/source/github/MyProject-build-yocto/—/usr/share/cmake-3.12/Modules/FindBoost.cmake:2048 (message):
| Unable to find the requested Boost libraries.
| Unable to find the Boost header files.

and farther down
| Boost_FOUND is 0
| Boost_INCLUDE_DIR is Boost_INCLUDE_DIR-NOTFOUND
| Boost_DIR is Boost_DIR-NOTFOUND
| Boost_CONFIG is
| Boost_VERSION is

Is there another way to link to the Boost libraries? Can there be incompatibilities between the cmake program and modules of another version?

You should consider to work with find_package(boost_filesystem 1.71.0 REQUIRED CONFIG)

and use target_link_libraries(… Boost::filesystem) to use imported boost targets.

But it works also with this version:

cmake_minimum_required(VERSION 3.16)
project(MyProject CXX)

# Options for boost
option(Boost_DEBUG "Set to ON to enable debug output from FindBoost." OFF)
option(Boost_DETAILED_FAILURE_MSG "Set to ON to get detailed information" OFF)

if(NOT DEFINED BOOST_ROOT)
    set(BOOST_ROOT $ENV{BOOST_ROOT})
endif()

if((${CMAKE_SYSTEM_NAME} STREQUAL "Linux") AND (NOT BOOST_ROOT))
    message(STATUS "setting boost_root for Linux")
    set(BOOST_ROOT "/usr/local")
endif()
message("BOOST_ROOT is ${BOOST_ROOT}") #2020-01-15

find_package(Boost 1.71.0 REQUIRED COMPONENTS system chrono thread filesystem)

message("Boost_FOUND is ${Boost_FOUND}") #2020-01-15
message("Boost_INCLUDE_DIRS is ${Boost_INCLUDE_DIRS}") #2020-01-15
message("Boost_LIBRARIES is ${Boost_LIBRARIES}") #2020-01-15
message("Boost_LIB_DIAGNOSTIC_DEFINITIONS is ${Boost_LIB_DIAGNOSTIC_DEFINITIONS}") #2020-01-15
message("Boost_DIR is ${Boost_DIR}") #2020-01-15
message("Boost_CONFIG is ${Boost_CONFIG}") #2020-01-15
message("Boost_VERSION is ${Boost_VERSION}") #2020-01-15
Claus-MBP:test clausklein$ cmake -G Ninja .
-- The CXX compiler identification is AppleClang 8.0.0.8000042
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
BOOST_ROOT is /usr/local/opt/boost
CMake Warning at /usr/local/opt/boost/lib/cmake/boost_system-1.72.0/libboost_system-variant-shared.cmake:64 (message):
  Target Boost::system already has an imported location
  '/usr/local/Cellar/boost/1.72.0/lib/libboost_system-mt.dylib', which will
  be overwritten with
  '/usr/local/Cellar/boost/1.72.0/lib/libboost_system.dylib'
Call Stack (most recent call first):
  /usr/local/opt/boost/lib/cmake/boost_system-1.72.0/boost_system-config.cmake:57 (include)
  /usr/local/opt/boost/lib/cmake/Boost-1.72.0/BoostConfig.cmake:120 (find_package)
  /usr/local/opt/boost/lib/cmake/Boost-1.72.0/BoostConfig.cmake:185 (boost_find_component)
  /opt/local/share/cmake-3.16/Modules/FindBoost.cmake:443 (find_package)
  CMakeLists.txt:18 (find_package)


CMake Warning at /usr/local/opt/boost/lib/cmake/boost_chrono-1.72.0/libboost_chrono-variant-shared.cmake:64 (message):
  Target Boost::chrono already has an imported location
  '/usr/local/Cellar/boost/1.72.0/lib/libboost_chrono-mt.dylib', which will
  be overwritten with
  '/usr/local/Cellar/boost/1.72.0/lib/libboost_chrono.dylib'
Call Stack (most recent call first):
  /usr/local/opt/boost/lib/cmake/boost_chrono-1.72.0/boost_chrono-config.cmake:57 (include)
  /usr/local/opt/boost/lib/cmake/Boost-1.72.0/BoostConfig.cmake:120 (find_package)
  /usr/local/opt/boost/lib/cmake/Boost-1.72.0/BoostConfig.cmake:185 (boost_find_component)
  /opt/local/share/cmake-3.16/Modules/FindBoost.cmake:443 (find_package)
  CMakeLists.txt:18 (find_package)


CMake Warning at /usr/local/opt/boost/lib/cmake/boost_filesystem-1.72.0/libboost_filesystem-variant-shared.cmake:64 (message):
  Target Boost::filesystem already has an imported location
  '/usr/local/Cellar/boost/1.72.0/lib/libboost_filesystem-mt.dylib', which
  will be overwritten with
  '/usr/local/Cellar/boost/1.72.0/lib/libboost_filesystem.dylib'
Call Stack (most recent call first):
  /usr/local/opt/boost/lib/cmake/boost_filesystem-1.72.0/boost_filesystem-config.cmake:57 (include)
  /usr/local/opt/boost/lib/cmake/Boost-1.72.0/BoostConfig.cmake:120 (find_package)
  /usr/local/opt/boost/lib/cmake/Boost-1.72.0/BoostConfig.cmake:185 (boost_find_component)
  /opt/local/share/cmake-3.16/Modules/FindBoost.cmake:443 (find_package)
  CMakeLists.txt:18 (find_package)


-- Found Boost: /usr/local/opt/boost/lib/cmake/Boost-1.72.0/BoostConfig.cmake (found suitable version "1.72.0", minimum required is "1.71.0") found components: system chrono thread filesystem 
Boost_FOUND is TRUE
Boost_INCLUDE_DIRS is /usr/local/Cellar/boost/1.72.0/include
Boost_LIBRARIES is Boost::system;Boost::chrono;Boost::thread;Boost::filesystem
Boost_LIB_DIAGNOSTIC_DEFINITIONS is 
Boost_DIR is /usr/local/opt/boost/lib/cmake/Boost-1.72.0
Boost_CONFIG is /usr/local/opt/boost/lib/cmake/Boost-1.72.0/BoostConfig.cmake
Boost_VERSION is 1.72.0
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/clausklein/tmp/test
Claus-MBP:test clausklein$ 

cmake -G Ninja -DCMAKE_PREFIX_PATH=$BOOST_ROOT …

works with

find_package(boost_filesystem 1.71.0 REQUIRED CONFIG)