RFC: use contrib/install.sh to install gmp

This replaces some `sed` patching (added in #7316) and Windows special-casing. Should be functionally equivalent.

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.

@staticfloat and @crayxt, can you take a look over this?

What's generally the purpose of the `touch` at the end of the `_OBJ_TARGET` rules? Openblas for example (which doesn't use autotools) doesn't have it, so none of the other large-ish dependencies need to recompile after `make cleanall`, they just need to reinstall which is quicker.

This tests fine on Linux and Windows, doing `make -C deps distclean-gmp distclean-mpfr && rm -rf usr` for good measure. Would appreciate testing on Mac - I put in another `*` before `$(SHLIB_EXT)` because I remember seeing somewhere that versioned dylibs put the numbers before the extension? Travis doesn't tell us too much for dependency build changes.