prefix.h: disable clock_gettime() and time() on MinGW builds
prefix.h: disable clock_gettime() and time() on MinGW builds

Let's try to use the native Win32 clock APIs.

Signed-off-by: Steven Noonan <4068f0880b399410602d694b3cc711c8a8f4727e@uplinklabs.net>

tycho/clockperf,tycho/clockperf