Add a configure option --enable-jhbuild-wrapper-script

The current gnome-shell.in script has a huge amount of
unnecessary complexity for the installed, normal case.  Fix
this by adding a configure option (defaulting to false) that
installs a simple, obvious wrapper script around mutter.

We do change the gnome-shell build setup to pass this option
by default for jhbuild.

https://bugzilla.gnome.org/show_bug.cgi?id=642084

tools/build/gnome-shell.modules

@@ -220,7 +220,7 @@
     </dependencies>
   </autotools>
 
-  <autotools id="gnome-shell">
+  <autotools id="gnome-shell" autogenargs="--enable-jhbuild-wrapper-script">
     <branch repo="git.gnome.org" module="gnome-shell"/>
     <dependencies>
         <dep package="gobject-introspection"/>