After 06ec9c47 we check for these
functions in configure (which will succeed in cygwin), but cmdutils.c
only includes windows.h if _WIN32 is defined (which it isn't in cygwin).

Retain the old intent from before 06ec9c47,
that these functions only would be used when _WIN32 is defined, while
only using them if configure has agreed that they do exist.
Signed-off-by: Martin Storsjö <martin@martin.st>