use contrib/install.sh to install gmp

this replaces some patching and windows special-casing.

removing the touch means mpfr no longer rebuilds every time after `make cleanall`
before: make cleanall && time(make -C deps install-mpfr) took 2 minutes,
after it takes one second
Merge pull request #8645 from JuliaLang/tk/gmp-install-adjust

RFC: use contrib/install.sh to install gmp