Add -pthread to extralibs and not to ldflags so that the dependency gets

reflected in the pkg-config files.
noticed by j .at. v2v .dot. cc

Originally committed as revision 11618 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1606,10 +1606,10 @@ if enabled pthreads; then
         :
     elif check_func pthread_create -pthread; then
         add_cflags -pthread
-        add_ldflags -pthread
+        add_extralibs -pthread
     elif check_func pthread_create -pthreads; then
         add_cflags -pthreads
-        add_ldflags -pthreads
+        add_extralibs -pthreads
     elif ! check_lib pthread.h pthread_create -lpthread; then
         die "ERROR: can't find pthreads library"
     fi