Comment 6 for bug 395978

Looks like the prototype changed between version 2.6 and 2.7. I added a line to the SConscript which performs a CheckLibWithHeader for libfaad and explicitly tries to compile the line:

faacDecInit2(0, 0, 0, (unsigned long*)0, (unsigned char*)0);

If the compilation fails, then we know that 2.6 is installed, so I define an M4AHACK symbol that we can #ifdef in mp4-mixxx.cpp.