[LyX/master] Make the missing class warning opt-out

[Juergen Spitzmueller]

commit af4090bb0a5936b178ff61e3e989e354f9e4c02a
Author: Juergen Spitzmueller <sp...@lyx.org>
Date:   Mon Mar 16 20:34:32 2015 +0100

    Make the missing class warning opt-out

diff --git a/src/BufferParams.cpp b/src/BufferParams.cpp
index 5cf529c..53ebf01 100644
--- a/src/BufferParams.cpp
+++ b/src/BufferParams.cpp
@@ -634,7 +634,7 @@ string BufferParams::readToken(Lexer & lex, string const & 
token,
                                                 "See section 3.1.2.2 (Class 
Availability) of the\n"
                                                 "User's Guide for more 
information."), desc, prereqs);
                        frontend::Alert::warning(_("Document class not 
available"),
-                                      msg);
+                                      msg, true);
                }
        } else if (token == "\\begin_preamble") {
                readPreamble(lex);