contrib: add --enable-pthreads/--enable-w32threads to ffmpeg

Should work in linux, no idea on win32 builds, so you're free to reverts
if it kills your kittens.

extras/contrib/src/Makefile

@@ -1048,7 +1048,9 @@ FFMPEGCONF += \
 	--disable-protocols \
 	--disable-network
 ifdef HAVE_WIN32
-FFMPEGCONF += --disable-bzlib --disable-decoder=dca --disable-encoder=ac3 --enable-libmp3lame
+FFMPEGCONF += --disable-bzlib --disable-decoder=dca --disable-encoder=ac3 --enable-libmp3lame --enable-w32threads
+else
+FFMPEGCONFG += --enable-pthreads
 endif
 
 ifdef HAVE_WINCE