[gnome-photos] build: Update glibtests.m4 from GLib

[Debarshi Ray <debarshir src gnome org>]

commit c04c4dab2681473f7ec690ddf323c38c5381c907
Author: Debarshi Ray <debarshir gnome org>
Date:   Mon Oct 30 12:04:57 2017 +0100

    build: Update glibtests.m4 from GLib
    
    Adds #serial to prevent an outdated copy from being used if multiple
    versions are available in the search path.

 m4/glibtests.m4 |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/m4/glibtests.m4 b/m4/glibtests.m4
index 7d5920a..581726a 100644
--- a/m4/glibtests.m4
+++ b/m4/glibtests.m4
@@ -1,3 +1,6 @@
+# Increment this whenever this file is changed.
+#serial 1
+
 dnl GLIB_TESTS
 dnl