Fix stdcall ABI call convention.

FFI_STDCALL is no define, so we can not use the preprocessor to check for it's presense. The result is, that stdcall is currently never used. This patch uses libffi's defines in order to enable the stdcall option.

This fixes issue #302 and adds a test case for stdcall callbacks.

This issue was resolved before in commit 1dcb818 (resulting to issue #213) but commit 32b1a4e broke it again.