Fix check_func_headers. The reference to the function under test was being
optimized out by gcc when the headers lied about the function existing,
rendering the test useless in that situation.
This new test throws a warning in mingw-w64, but that should not be a problem.
Patch by Martin Storsjö <martin at martin dot st>

Originally committed as revision 19440 to svn://svn.ffmpeg.org/ffmpeg/trunk
