[evince/wip/hadess/fix-meson-build] help: Fix meson build with shell disabled

[Bastien Nocera <hadess src gnome org>]

commit 5c6e3e0b010abb816c39a1a2ac35a8d6bc373f0d
Author: Bastien Nocera <hadess hadess net>
Date:   Thu Feb 28 16:46:59 2019 +0100

    help: Fix meson build with shell disabled
    
    $ meson -Dviewer=false -Dgtk_doc=true _build
    <snip>
    help/reference/shell/meson.build:21:6: ERROR:  Unknown variable "shell_inc".

 help/meson.build | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/help/meson.build b/help/meson.build
index 9646ab4f..19cd0c83 100644
--- a/help/meson.build
+++ b/help/meson.build
@@ -13,7 +13,9 @@ if enable_gtk_doc
 
   subdir('reference/libdocument')
   subdir('reference/libview')
-  subdir('reference/shell')
+  if enable_viewer
+    subdir('reference/shell')
+  endif
 endif
 
 sources = [