Build using --with-gmp and shared libraries

[François-Xavier Coudert]

>>> Testing done on i686-linux (built with --languages=c,fortran and
>>> a shared libgmp in /foo/bar, and regtested).
>>> OK for mainline? OK for 4.0?

ping**3, build machinery maintainers in Cc.

This patch makes --with-gmp and --with-mpfr similar to --with-as and
others, where you don't need to have the as program in your PATH if
explicitly specified.

FX