Fix feature macros

We shouldn't be using _POSIX_C_SOURCE and friends mid-file; it's documented not to work in man 7 features, and doesn't because if glibc-internal header features.h already got included somewhere and set the #defines it sets on the basis of it, they won't get set again because of multiple include guards. We should be using things like __USE_ISOC11 as proposed in !40 (closed) even less; that's one of the glibc internal flags being set from features.h, not part of the public API, and may just break on future glibc versions or other libcs.

Note: this splits out the compiler flags in SConstruct by Macness, but I don't have access to a Mac for testing and we should revisit it when we do.