[gtk-osx] Fix bootstrap on 10.11

[John Ralls <jralls src gnome org>]

commit 7926a99ac9081ca4c4e9e2a2e8b8973bfeb217d5
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Wed Feb 17 16:16:40 2016 +0100

    Fix bootstrap on 10.11
    
    During jhbuild bootstrap there is not "bootstrap" module in modules here
    and CONFIG_SHELL gets set to a non-existing bash.
    Instead set CONFIG_SHELL only if the shell binary exists.

 jhbuildrc-gtk-osx |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/jhbuildrc-gtk-osx b/jhbuildrc-gtk-osx
index bd60705..95e93a7 100644
--- a/jhbuildrc-gtk-osx
+++ b/jhbuildrc-gtk-osx
@@ -368,10 +368,11 @@ def setup_sdk(target, sdk_version, architectures=[_default_arch]):
     # El Capitan needs bash to work around SIP. If you're using a
     # common bootstrap directory (e.g. $HOME/.local) then override
     # CONFIG_SHELL in .jhbuildrc-custom after calling setup_sdk().
+    config_shell = os.path.join(prefix, 'bin', 'bash')
     if _osx_version < 11.0:
         skip.append('bash')
-    elif not 'bootstrap' in modules:
-        os.environ['CONFIG_SHELL'] = os.path.join(prefix, 'bin', 'bash')
+    elif os.path.exists(config_shell):
+        os.environ['CONFIG_SHELL'] = config_shell
 
     # gettext-fw rebuilds gettext with an in-tree libiconv to get
     # around the Apple-provided one not defining _libiconv_init for