Makefile: Fix regression in target-bound custom command COMMENT output

Fix a logic typo introduced by commit v3.1.0-rc1~781^2 (Generalize
cmCustomCommandGenerator to more fields, 2014-03-10).

Source/cmLocalUnixMakefileGenerator3.cxx

@@ -1076,7 +1076,7 @@ cmLocalUnixMakefileGenerator3
   if(echo_comment)
     {
     const char* comment = ccg.GetComment();
-    if(comment && !*comment)
+    if(comment && *comment)
       {
       this->AppendEcho(commands, comment,
                        cmLocalUnixMakefileGenerator3::EchoGenerate);