Bug 1341234 - part 3 - add various system library CFLAGS to BINDGEN_SYSTEM_FLAGS; r=gps

This change ensures that folks who configure --with-system-FOO for
various values of FOO can build Stylo, since bindgen will know where to
find the flags for said FOO packages.

old-configure.in

@@ -4395,7 +4395,7 @@ AC_SUBST_LIST(MOZ_CAIRO_LIBS)
 AC_SUBST_LIST(MOZ_CAIRO_OSLIBS)
 AC_SUBST(MOZ_TREE_PIXMAN)
 
-BINDGEN_SYSTEM_FLAGS="$_BINDGEN_CFLAGS"
+BINDGEN_SYSTEM_FLAGS="$_BINDGEN_CFLAGS $NSPR_CFLAGS $NSS_CFLAGS $MOZ_PIXMAN_CFLAGS $MOZ_CAIRO_CFLAGS"
 AC_SUBST_TOML_LIST(BINDGEN_SYSTEM_FLAGS)
 
 dnl ========================================================