[LyX/master] Fix bug #8830: Compile error with disabled NLS

[Vincent van Ravesteijn]

commit 72f70cbe5e34e5a34b3b43250542e47ae73b1a04
Author: Vincent van Ravesteijn <v...@lyx.org>
Date:   Sun Sep 8 18:00:36 2013 +0200

    Fix bug #8830: Compile error with disabled NLS
    
    The Messages::gui_lang_ variable is instantiated in the '#ifdef ENABLE_NLS' 
block. To prevent compile problems, we should also instantiate it when NLS is 
disabled.

diff --git a/src/support/Messages.cpp b/src/support/Messages.cpp
index 906ec3b..79bc141 100644
--- a/src/support/Messages.cpp
+++ b/src/support/Messages.cpp
@@ -305,6 +305,8 @@ docstring const Messages::get(string const & m) const
 
 namespace lyx {
 
+std::string Messages::gui_lang_;
+
 Messages::Messages(string const & /* l */) {}
 
 docstring const Messages::get(string const & m) const