Add lines to the sources.exclude file to select paths and wildcards
that will be excluded from source tarballs. See the tar
--exclude-from=FILE option for syntax.
If a source directory that is used to build targets is included then
those targets should be defined in a CMakeLists.txt file in the same
directory and the following syntax should be used in the parent
CMakeLists.txt script to ensue the parent CMakeLists.txt file remains
valid when the excluded directory is not present:
if (EXITS ${CMAKE_SOURCE_DIR}/optional-source-dir AND IS_DIRECTORY ${CMAKE_SOURCE_DIR}/optional-source-dir)
add_subdirectory (optional-source-dir)
endif ()
