cmake HAS_EXP_17_FS_WITH_STDC examples

Wargus/wargus CMakeLists.txt :213 

check_cxx_source_compiles("${EXPERIMENTAL_FS_SRC}" HAS_EXP_17_FS_WITH_STDC)

Wargus/stratagus CMakeLists.txt :632 

check_cxx_source_compiles("${EXPERIMENTAL_FS_SRC}" HAS_EXP_17_FS_WITH_STDC)