g_test_build_filename() API documentation still mentions Makefile.decl, but
it has been renamed to glib.mk in
f9eb9eed
https://bugzilla.gnome.org/show_bug.cgi?id=722436
*
* In order for this function to work in srcdir != builddir situations,
* the G_TEST_SRCDIR and G_TEST_BUILDDIR environment variables need to
- * have been defined. As of 2.38, this is done by the Makefile.decl
+ * have been defined. As of 2.38, this is done by the glib.mk
* included in GLib. Please ensure that your copy is up to date before
* using this function.
*