CMake Nightly Date Stamp
GenerateExportHeader: Deprecate add_compiler_export_flags function.

This is made obsolete by the CXX_VISIBILITY_PRESET and
VISIBILITY_INLINES_HIDDEN target properties.
Merge topic 'deprecate-add_compiler_export_flags' into next

0f3a666 GenerateExportHeader: Deprecate add_compiler_export_flags function.
ce6c736 CMake Nightly Date Stamp