Fix missing include of windows.h on windows platform. Change check
for _POSIX_TIMERS to check that it is greater than 0. On OS X, the
POSIX timers are not available and _POSIX_TIMERS is -1.
w32gdi.c: Implements gamma ramp method for Windows GDI API.
Use double as a cross platform time representation.
Add WinGDI as a selectable method (currently limitied to minimum 4500K).
Fix a bug where redshift would crash if RANDR failed and VidMode wasn't
compiled in.