GCC has added support for new `-std=` options: * `-std=c17` added by [r254216](https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=254216) (Add -std=c17, -std=gnu17, 2017-10-30) * `-std=c18` added by [r254847](https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=254847) (Add -std=c18 etc. option aliases, 2017-11-16) This is in anticipation of a [new C standard ISO/IEC FDIS 9899](https://www.iso.org/standard/74528.html). Currently it is unknown whether it will be called C17 or C18. Once this is known, CMake will need to be taught about the new C standard level. See also #17740.