Enable swscale in MinGW vhooks only if swscale was configured.

Copied from Cygwin configure.

Originally committed as revision 10073 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1163,8 +1163,12 @@ case $targetos in
     targetos=mingw32
     shlibdir="$bindir"
     dv1394="no"
-    VHOOKSHFLAGS='-shared -L$(BUILD_ROOT)/libavformat -L$(BUILD_ROOT)/libavcodec -L$(BUILD_ROOT)/libswscale -L$(BUILD_ROOT)/libavutil'
-    VHOOKLIBS='-lavformat$(BUILDSUF) -lavcodec$(BUILDSUF) -lswscale$(BUILDSUF) -lavutil$(BUILDSUF) $(EXTRALIBS)'
+    VHOOKSHFLAGS='-shared -L$(BUILD_ROOT)/libavformat -L$(BUILD_ROOT)/libavcodec -L$(BUILD_ROOT)/libavutil'
+    VHOOKLIBS='-lavformat$(BUILDSUF) -lavcodec$(BUILDSUF) -lavutil$(BUILDSUF) $(EXTRALIBS)'
+    if enabled swscaler; then
+        VHOOKSHFLAGS="$VHOOKSHFLAGS -L\$(BUILD_ROOT)/libswscale"
+        VHOOKLIBS="$VHOOKLIBS -lswscale\$(BUILDSUF)"
+    fi
     ffserver="no"
     network="no"
     SLIBPREF=""