[gvfs] build: Fix UDisks2 build when gudev is disabled

[Iñigo Martínez <inigomartinez src gnome org>]

commit 8cd3e024e42d6aa22a97410a382b1da4d44c1182
Author: Iñigo Martínez <inigomartinez gmail com>
Date:   Tue Jan 23 21:07:48 2018 +0100

    build: Fix UDisks2 build when gudev is disabled
    
    UDisks2 option needs gudev because it makes use of it.
    
    This patch shows an error message in case this options is enabled
    but gudev option is disabled.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=786149

 meson.build |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/meson.build b/meson.build
index 39ea8c1..2c8a29a 100644
--- a/meson.build
+++ b/meson.build
@@ -362,6 +362,7 @@ endif
 # *** Check for udisks2 ***
 enable_udisks2 = get_option('udisks2')
 if enable_udisks2
+  assert(enable_gudev, 'UDisks2 requested but gudev is required')
   udisks2_dep = dependency('udisks2', version: '>= 1.97')
 endif