[glib-networking/wip/nacho/gnutls-optional] meson: error out if no TLS backends are enabled

[Ignacio Casal Quinteiro <icq src gnome org>]

commit abcd97bd5a76a3f89ed897c2a6858806ede47e12
Author: Ignacio Casal Quinteiro <qignacio amazon com>
Date:   Tue Sep 11 15:29:48 2018 +0200

    meson: error out if no TLS backends are enabled

 meson.build | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/meson.build b/meson.build
index 9e6ef6f..17a2779 100644
--- a/meson.build
+++ b/meson.build
@@ -85,6 +85,10 @@ if gnutls_dep.found()
   endif
 endif
 
+if backends.length() == 0
+  error('No TLS backends enabled. Please enable at least one TLS backend')
+endif
+
 configure_file(
   output: 'config.h',
   configuration: config_h