[Makefile] Rebuild .def files if config changed

I was getting check-symbols failure because my previous build was
without CoreText, and after reconfiguring with CoreText, the old
harfbuzz.defs file was not being regenerated.

src/Makefile.am

@@ -235,6 +235,7 @@ DEF_FILES += harfbuzz-gobject.def
 endif
 check: $(DEF_FILES) # For check-symbols.sh
 CLEANFILES += $(DEF_FILES)
+harfbuzz.def: $(top_builddir)/config.status
 harfbuzz.def: $(HBHEADERS)
 	$(AM_V_GEN) $(srcdir)/gen-def.py "$@" $^
 harfbuzz-subset.def: $(HB_SUBSET_headers)