These functions are available in time.h (conditional on posix thread
safe functions) on mingw.

Previously, these functions weren't detected by configure, and
libavutil/time_internal.h provided replacements, even if time.h
actually contained definitions of them.

These mingw inline functions are currently defined as
 "extern __inline __attribute__((__gnu_inline__))". In this case,
redefining a new static inline version of the same function with the
same name is tolerated.

Signed-off-by: Martin Storsjö <martin@martin.st>