Bug in gmpbench-0.2 when building under Windows with msys/MinGW(32 or 64)

[Lance W. Patterson]

The check of which header files to include for the cputime() function does not properly check for MinGW on Windows, so it will include the headers from "#else".  This is in every source benchmark file.

The following modification will address that problem.

#if !defined (__sun) \
    && (defined (USG) || defined (__SVR4) || defined (_UNICOS) \
	|| defined (__hpux) \
	|| (defined(__WIN32__) && defined(__MINGW32__)))

"__WIN32__" and "__MINGW32__" are defined for both "MinGW/msys" and "MinGW64/msys".  This is not an issue with Cygwin nor should the update affect it.