Fix signed/unsigned comparison in EscapeJSON

Source/cmGlobalUnixMakefileGenerator3.cxx

@@ -142,7 +142,7 @@ void cmGlobalUnixMakefileGenerator3
 //----------------------------------------------------------------------------
 std::string EscapeJSON(const std::string& s) {
   std::string result;
-  for (int i = 0; i < s.size(); ++i) {
+  for (std::string::size_type i = 0; i < s.size(); ++i) {
     if (s[i] == '"' || s[i] == '\\') {
       result += '\\';
     }