This makes the user able to use the familiar `make install` syntax, and
allows overriding of the default directories using the DESTDIR and
PREFIX variables, for example:
 make DESTDIR=packaging PREFIX=/usr install

This provides users of this library the guarantee that the ABI will
stay stable when MAJOR will reach 1, and will stay backwards compatible
for the entire duration of the 1.x.y branch.

It does require the maintainers to always update the version in the
Makefile at every ABI change.

sha256.c and aes.c contain conflicting declarations, so we need to compile them
as separate units. This requires a bit more Makefile-shuffling; the build
directory now includes 'src' or 'lib' as appropriate, and we just mkdir -p
before each compilation.

think this got lost when mark added separate release/debug builds

Some of the crypto libs rely on UINT64_C, which in glibc 2.17 and earlier was
not defined for C++ code (see
https://sourceware.org/bugzilla/show_bug.cgi?id=15366).

Builds fuzzers using http://lcamtuf.coredump.cx/afl/

So that we can build everything together.

We want to stop aes_* and sha_* functions leaking out of our .so, as well as
internal _olm_* symbols.

This also means we need to link the unit tests against the objects. Possibly we
should distinguish between unit tests and integration tests.

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