Qt4 - Try to fix the detection of video tracks and show of related buttons, fix a segfault that can happen.

contrib/libcaca: combining 'macosx-version-min=10.4' with the 10.3.9 SDK isn't exactly a great idea. This patch strips 10.3.9 support from libcaca and compiles it with GCC 4, etc. on PowerPC-based Macs, too.

strnlen: export its VLC pendant properly and check its existance in cc.c. Fixes the compilation onDarwin

Now vlc/vlc.h can be included out of the box.
However, I believe it will only work properly on little-endian 32-bits platforms.

This fixes the single largest source of problems when including vlc/*.h out of the box.

Standard MIDI File: don't abuse demux_vaControlHelper which is meant for sample-based audio files. This fixes the warping position and prevent uninplemented seeking.