cxxmodules: give a reason for unavailability of `import std`
If the CMAKE_CXX<STANDARD>_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE
variable is set, include its value in the error message.
Cc: @bill-hoffman
Edited by Ben Boeckel
If the CMAKE_CXX<STANDARD>_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE
variable is set, include its value in the error message.
Cc: @bill-hoffman