glib: Add installed Valgrind suppressions file for GLib and GIO

While we cannot get Valgrind to automatically load this suppression file
for applications which link to GLib
(https://bugs.kde.org/show_bug.cgi?id=160905), we can at least install
it on systems in a shared directory, so that developers can use a
standardised (and up-to-date) suppressions file for GLib, rather than
rolling their own.

The file will typically be installed to:
   /usr/share/glib-2.0/valgrind/glib.supp

Distributors: it is recommended that this suppression file be installed
as part of the development package for GLib in your distribution.

https://bugzilla.gnome.org/show_bug.cgi?id=666114

Makefile.am

@@ -103,6 +103,10 @@ endif
 
 $(pkgconfig_DATA): config.status
 
+# Install Valgrind suppression file
+valgrinddir = $(datadir)/glib-2.0/valgrind
+dist_valgrind_DATA = glib.supp
+
 # build documentation when doing distcheck
 DISTCHECK_CONFIGURE_FLAGS = --enable-debug --enable-gtk-doc --enable-man --disable-maintainer-mode