Create swscale subdirs for out-of-tree builds.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@18191 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -2507,6 +2507,11 @@ if enabled source_path_used; then
         libavutil         \
         libpostproc       \
         libswscale        \
+        libswscale/bfin   \
+        libswscale/mlib   \
+        libswscale/ppc    \
+        libswscale/sparc  \
+        libswscale/x86    \
         tests             \
         tools             \
         "