app: build layer mode operations with SSE2 flags.

Some of the generic files still contain SSE2 code, in particular
gimpoperationlayermode.c. The reason why it often works without is that
native gcc will usually pre-define SSE macros by default.
To check this: gcc -dM -E - < /dev/null | grep SSE
Yet I had a case on a small netbook where the SSE macros were not
pre-defined even though supported. Consequently the build failed.

app/operations/layer-modes/Makefile.am

@@ -9,6 +9,7 @@ AM_CPPFLAGS = \
 	$(CAIRO_CFLAGS)				\
 	$(GEGL_CFLAGS)				\
 	$(GDK_PIXBUF_CFLAGS)			\
+	$(SSE2_EXTRA_CFLAGS)		\
 	-I$(includedir)
 
 noinst_LIBRARIES = \