mp4ff: use faacDecInit2() to find the AAC track

Use faacDecInit2() instead of AudioSpecificConfig() to detect the AAC
track in the MP4 file.  This has a great advantage: it initializes the
libfaad decoder, which the caller would normally do anyway - but now
we can go without the AudioSpecificConfig() call.  When decoder==NULL
(called from mp4_tag_dup()), fall back to a mp4ff_get_track_type()==1
check, like other audio players do.

configure.ac

@@ -812,7 +812,6 @@ int main() {
 		])
 	fi
 	if test x$enable_aac = xyes; then
-		AC_CHECK_TYPES(mp4AudioSpecificConfig,,,[#include <faad.h>])
 		AC_CHECK_MEMBERS([faacDecConfiguration.downMatrix,faacDecConfiguration.dontUpSampleImplicitSBR,faacDecFrameInfo.samplerate],,,[#include <faad.h>])
 		AC_DEFINE(HAVE_FAAD,1,[Define to use FAAD2 for AAC decoding])
 	else