For reference, here’s my toolchain file. Setting CMAKE_INCLUDE_PATH et al didn’t work.
#
# CMake Toolchain file for crosscompiling on ARM.
#
# Target operating system name.
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR aarch64)
set(CMAKE_CROSSCOMPILING TRUE)
# Name of C compiler.
set(CMAKE_C_COMPILER "/usr/bin/aarch64-linux-gnu-gcc-8")
set(CMAKE_CXX_COMPILER "/usr/bin/aarch64-linux-gnu-g++-8")
# Where to look for the target environment. (More paths can be added here)
set(CMAKE_FIND_ROOT_PATH /usr/aarch64-linux-gnu)
set(CMAKE_INCLUDE_PATH /usr/include/aarch64-linux-gnu)
set(CMAKE_LIBRARY_PATH /usr/lib/aarch64-linux-gnu)
set(CMAKE_PROGRAM_PATH /usr/bin/aarch64-linux-gnu)
# Adjust the default behavior of the FIND_XXX() commands:
# search programs in the host environment only.
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
# Search headers and libraries in the target environment only.
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)