[glib] meson: Fix libmount support

[Philip Withnall <pwithnall src gnome org>]

commit 69b2b5f262e8ddcacf20fa0518e83b9679b6db82
Author: Jan Alexander Steffens (heftig) <jan steffens gmail com>
Date:   Tue Oct 31 04:11:42 2017 +0100

    meson: Fix libmount support
    
    The define was still missing.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=789681

 config.h.meson |    3 +++
 meson.build    |    1 +
 2 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/config.h.meson b/config.h.meson
index c67e179..3f310b3 100644
--- a/config.h.meson
+++ b/config.h.meson
@@ -247,6 +247,9 @@
 /* Define if libelf is available */
 #mesondefine HAVE_LIBELF
 
+/* Define if libmount is available */
+#mesondefine HAVE_LIBMOUNT
+
 /* Define to 1 if you have the <linux/magic.h> header file. */
 #mesondefine HAVE_LINUX_MAGIC_H
 
diff --git a/meson.build b/meson.build
index 208c230..cd9cbf2 100644
--- a/meson.build
+++ b/meson.build
@@ -1535,6 +1535,7 @@ if use_system_pcre
 endif
 if libmount_dep.length() == 1 and libmount_dep[0].found()
   glib_conf.set('LIBMOUNT_LIBS', '-lmount')
+  glib_conf.set('HAVE_LIBMOUNT', 1)
 endif
 glib_conf.set('GIO_MODULE_DIR', '${libdir}/gio/modules')
 # FIXME: Missing: