Check*CompilerFlag: Document use of CMAKE_REQUIRED_DEFINITIONS (#14309)

Explain how CMAKE_REQUIRED_DEFINITIONS is set before calling the
check_*_source_compiles macros.
Merge topic 'doc-check-compiler-flag-override' into next

3e79d65 Check*CompilerFlag: Document use of CMAKE_REQUIRED_DEFINITIONS (#14309)
Genex: Allow relative paths in INSTALL_INTERFACE.

These paths can be prepended with the ${_IMPORT_PREFIX} generated
in the export file.

Such relative paths were previously an error.
Merge topic 'install-interface-relative' into next

d777b8e Genex: Allow relative paths in INSTALL_INTERFACE.