1. BORG_*_PREFIX is checked (avoids lib detection via pkg-config).
2. pkg-config is tried
3. fallback to bundled C code (or failure in case of OpenSSL)
also:
- simplified code again
- removed (c) headers, nothing left of original code
- removed hardcoded lib search pathes
- to find system libs/headers, one must point these env vars to them:
BORG_OPENSSL_PREFIX
- moved some code from setup.py to setup_*.py