[gcalctool] Allow underscores in variable names

[Robert Ancell <rancell src gnome org>]

commit 85146bf4a757a037e8a1156f7e4764bb3b401903
Author: PioneerAxon <arth svnit gmail com>
Date:   Sun Oct 14 20:01:10 2012 +1300

    Allow underscores in variable names

 src/equation-lexer.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/equation-lexer.vala b/src/equation-lexer.vala
index a14c83a..f012168 100644
--- a/src/equation-lexer.vala
+++ b/src/equation-lexer.vala
@@ -147,7 +147,7 @@ public class PreLexer
         if (c == '"')
             return LexerTokenType.PL_SECOND;
 
-        if (c.isalpha ())
+        if (c.isalpha () || c == '_')
             return LexerTokenType.PL_LETTER;
 
         if (c == 'â')