gnu: glib-networking: Prefer 'SSL_CERT_FILE' as TLS CA file.

This is an improvement over commit af6100f.

* gnu/packages/gnome.scm (glib-networking)[source](patches): New field.
  [arguments]: Pass '/etc/ssl/certs/ca-certificates.crt' to configure.
  Set 'SSL_CERT_FILE' to '/dev/null' in 'use-empty-ssl-cert-file' phase.
  (libsoup)[arguments]: Set 'SSL_CERT_FILE' to '/dev/null' in
  'pre-check' phase.
* gnu/packages/patches/glib-networking-ssl-cert-file.patch: New patch.
* gnu-system.am (dist_patch_DATA): Add it.

gnu/packages/patches/glib-networking-ssl-cert-file.patch

@@ -0,0 +1,29 @@
+From b010e41346d418220582c20ab8d7f3971e4fb78a Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?=E5=AE=8B=E6=96=87=E6=AD=A6?= <iyzsong@gmail.com>
+Date: Fri, 14 Aug 2015 17:28:36 +0800
+Subject: [PATCH] gnutls: Allow overriding the anchor file location by
+ 'SSL_CERT_FILE'
+
+---
+ tls/gnutls/gtlsbackend-gnutls.c | 4 +++-
+ 1 file changed, 3 insertions(+), 1 deletion(-)
+
+diff --git a/tls/gnutls/gtlsbackend-gnutls.c b/tls/gnutls/gtlsbackend-gnutls.c
+index 55ec1a5..217d3c8 100644
+--- a/tls/gnutls/gtlsbackend-gnutls.c
++++ b/tls/gnutls/gtlsbackend-gnutls.c
+@@ -101,8 +101,10 @@ g_tls_backend_gnutls_real_create_database (GTlsBackendGnutls  *self,
+                                            GError            **error)
+ {
+   const gchar *anchor_file = NULL;
++  anchor_file = g_getenv ("SSL_CERT_FILE");
+ #ifdef GTLS_SYSTEM_CA_FILE
+-  anchor_file = GTLS_SYSTEM_CA_FILE;
++  if (!anchor_file)
++    anchor_file = GTLS_SYSTEM_CA_FILE;
+ #endif
+   return g_tls_file_database_new (anchor_file, error);
+ }
+-- 
+2.4.3
+