POSIX.1-2008 deprecated gettimeofday, which we used as a fallback if the
monotonic clock was unavailable.
This replaces that fallback with the non-monotonic real-time clock.
Because of the Gordian knot of feature test macros across Unices, this
also includes the shift from some platform source files defining
_POSIX_C_SOURCE to various values to _DEFAULT_SOURCE being defined for
all source files on Linux. This is because -std=c99 on Linux disables
_DEFAULT_SOURCE (POSIX 2008 and extensions).
Once runtime platform selection comes in, this kind of platform-specific
preprocessor logic can be moved into the platform glue files and won't
need to be replicated by third-party build setups, but for now, sorry.
Inclusion of internal headers is already both centralized and follows
strict rules. Inclusion guards are both an unneccessary maintenance
burden and may hide inclusion order bugs.
