Fix loading of libffi on OpenBSD

Use short library name 'libffi.so' for OpenBSD. Libffi package might change the version number but the library should work without the full "libffi.so.0.0" suffix.

In addition, add compiler flag to include /usr/local/include to header search path. These are not added by default in OpenBSD. Otherwise libffi fails to find /usr/local/include/ffi.h.