Add support for C17 or C18

GCC has added support for new -std= options:

• -std=c17 added by r254216 (Add -std=c17, -std=gnu17, 2017-10-30)
• -std=c18 added by r254847 (Add -std=c18 etc. option aliases, 2017-11-16)

This is in anticipation of a new C standard ISO/IEC FDIS 9899. 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 (closed).