[PATCH 0 of 1] Enable building on MSYS/Cygwin shell using native GCC when configure executed via abs path.

[Ray Donnelly]

The following patch for GMP 5.1.2 works around path namespace
differences between MSYS/Cygwin and native Windows GCC in configure by
calculating and using relative paths instead of absolute ones.

I used some macros from the autoconf-archive project for the hard part
of determining the relative path from one folder to another (builddir
to srcdir).

Best regards,

Ray Donnelly.