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.
CLOCK_MONOTONIC should not be used as a feature macro. The POSIX
feature macros are provided by unistd.h. CLOCK_MONOTONIC is provided by
time.h. CLOCK_MONOTONIC requires _POSIX_C_SOURCE >= 199309L on some
systems.
