Contribs: make libass static on all platforms

We only link it once, so this is more efficient. This allows the build
-time linker to remove unused functions and the export table.

We could probably do the same with a few other libraries.

extras/contrib/src/Makefile

@@ -2442,11 +2442,7 @@ libass:
 	(cd $@; autoreconf -ivf)
 
 .libass: libass .freetype
-ifdef HAVE_DARWIN_OS
 	(cd $<; $(HOSTCC) ./configure --disable-png --disable-shared $(HOSTCONF) --prefix=$(PREFIX) CFLAGS="$(CFLAGS) -O3" && make && make install)
-else
-	(cd $<; $(HOSTCC) ./configure --disable-png $(HOSTCONF) --prefix=$(PREFIX) CFLAGS="$(CFLAGS) -O3" && make && make install)
-endif
 	touch $@
 
 CLEAN_FILE += .libass