build: don't export _POSIX_C_SOURCE on non-linux

Don't export _POSIX_C_SOURCE to dependents. On the BSDs (and the
BSD-ish, like OS X), it hides SysV and BSD definitions.

The exception is Linux: _POSIX_C_SOURCE=200112 unlocks some of the
newer pthreads features like spinlocks and barriers, so keep
exporting it on that platform.