Add patch for compiling GMP 6.0.0 on OSX
Remove gmp-5.1.3 checksums
Avoid warning for possibly non-existing file

Also remove tab to avoid emacs warning about makefile syntax

Squashed the 4 commits from #8192

closes #8192
Merge branch 'make-warning'
Merge pull request #7957 from JuliaLang/sf/gmp6osx

[WIP] Update gmp to 6.0.0
Merge branch 'JuliaLang-master'