Increase minimum C11 compatible compiler

[ryandesign (Ryan Carsten Schmidt)]

MacPorts base thinks clang 500 / Xcode 5 and later are C11 compatible but only clang 700 / Xcode 7 and later actually are. See #62687, #60429.

[ryandesign (Ryan Carsten Schmidt)]

​According to wikipedia implementations are not required to support complex.h, stdatomic.h, and threads.h. So I don't know. Should we up the minimum C11 compiler to clang 700, since that added stdatomic.h which several ports require, or should ports that require it be forced to blacklist clang < 700 manually?

[chrstphrchvz (Christopher Chavez)]

Note that there are some ports which claim statomic.h is available as of Xcode clang 602 (Xcode 6.3) rather than Xcode clang 700 (Xcode 7.0).

One alternative might be to add additional options compiler.requires_stdatomic_h etc. (similar to compiler.thread_local_storage) to further restrict compiler selection. I‘m not certain this would be less total work to maintain, and awareness/adoption of it (among other things documented at wiki:CompilerSelection) likely depends on port maintainers’ awareness.