[Mingw-cross-env-list] gcc-mpfr update

[Mark Brand]

Hi,

GNU MPFR 3.1.0 changed some file locations [1]. I added some configure 
options [2] to gcc.mk to accommodate the new locations.

This seems to work fine, but I am not sure if this is the most elegant 
way of handling this.

[1] http://www.mpfr.org/mpfr-current/#changes
[2] http://hg.savannah.gnu.org/hgweb/mingw-cross-env/rev/8d4536e70c6e

regards,

Mark