Squashed 'deps/rust-gmp/' content from commit a0544cc

git-subtree-dir: deps/rust-gmp
git-subtree-split: a0544cc11674b0d4d036660ed912e2ea92d3c601
Merge commit '273995bda1ce488a36d601a370b3493e4fce9ff7' as 'deps/rust-gmp'
Update Makefile

More generic. Works with deps.