Inconsistencies with install() and relative path to GNUInstallDirs

Code
6

Do you mean this strange behaviour:

bash-5.2$ cmake --preset default --fresh
Preset CMake variables:

  BUILD_SHARED_LIBS="NO"
  CMAKE_BUILD_TYPE="Release"
  CMAKE_CXX_STANDARD="20"
  CMAKE_DEBUG_POSTFIX="D"
  CMAKE_INSTALL_PREFIX="/usr"
  CMAKE_PREFIX_PATH:STRING="/Users/clausklein/Workspace/cpp/ModernCmakeStarter/stagedir"

Preset environment variables:

  CPM_USE_LOCAL_PACKAGES="YES"
  
-- The CXX compiler identification is AppleClang 16.0.0.16000026
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found Git: /usr/local/bin/git (found version "2.46.2")
-- CPM: Adding package project_options@0.30.0 (v0.30.0 at /Users/clausklein/.cache/CPM/project_options/2de06ed3a4c11a5a7ebda8b90d2dd05882bbb095)
-- CPM: Adding package PackageProject.cmake@1.10.0 (v1.10.0 at /Users/clausklein/.cache/CPM/packageproject.cmake/41b1a5028ad2c8d2c6bb64eb6beff8b090d304e5)
-- CPM: Using local package fmt@10.0.0
-- Developer mode is OFF. For developement, use `-DENABLE_DEVELOPER_MODE:BOOL=ON`. Building the project for the end-user...
-- The default CMAKE_C_STANDARD used by external targets and tools is not set yet. Using the latest supported C standard that is 90
-- /usr/local/bin/ccache found and enabled
-- Configuring done (0.5s)
-- Generating done (0.0s)
-- Build files have been written to: /Users/clausklein/Workspace/cpp/ModernCmakeStarter/build/user
bash-5.2$ DESTDIR=/tmp/test cmake --install build/user              
-- Install configuration: "Release"
-- Up-to-date: /tmp/test/usr/etc/test.cfg
-- Up-to-date: /tmp/test/usr/include/greeter
-- Up-to-date: /tmp/test/usr/include/greeter/greeter
-- Up-to-date: /tmp/test/usr/include/greeter/greeter/version.h
-- Installing: /tmp/test/usr/lib/greeter/libgreeter.a
-- Installing: /tmp/test/usr/lib/cmake/greeter/greeterTargets.cmake
-- Installing: /tmp/test/usr/lib/cmake/greeter/greeterTargets-release.cmake
-- Up-to-date: /tmp/test/usr/lib/cmake/greeter/greeterConfigVersion.cmake
-- Up-to-date: /tmp/test/usr/lib/cmake/greeter/greeterConfig.cmake
-- Up-to-date: /tmp/test/usr/include/greeter
-- Up-to-date: /tmp/test/usr/include/greeter/greeter
-- Up-to-date: /tmp/test/usr/include/greeter/greeter/greeter.h
bash-5.2$ sudo cmake --install build/user 
Password:
-- Install configuration: "Release"
CMake Error at build/user/cmake_install.cmake:41 (file):
  file cannot create directory: /usr/etc.  Maybe need administrative
  privileges.

bash-5.2$