Now that we have C and C++, we need to split the compile and link steps
(because we need different flags for the C and C++ files), so this is
easier with a Makefile.

Replace the python scripts for building the shared lib and tests with a
Makefile, which makes it easier to handle a mix of C and C++.

git-subtree-dir: lib/curve25519-donna
git-subtree-split: 28772f37a4b8a57ab9439b9e79b19f9abee686da