build_cross_compiler.sh requests an invalid URL when fetching mpfr

[Herbstein]

The script uses http://www.mpfr.org/mpfr-current/mpfr-3.1.2.tar.xz to download mpfr, which is a url that isn't used anymore. The up-to-date url for the same version is http://www.mpfr.org/mpfr-3.1.2/mpfr-3.1.2.tar.xz.