Avoid infinite recursion between wxStockGDI and wxSystemSettions

There was a possible recursion from wxStockGDI::GetFont to
wxSystemSettings::GetFont and back, resolve by using direct font
creation, adding fixed system font.

src/osx/carbon/font.cpp

@@ -473,6 +473,8 @@ wxFont::wxFont(wxOSXSystemFont font)
         case wxOSX_SYSTEM_FONT_VIEWS:
             uifont = kCTFontViewsFontType;
             break;
+        case wxOSX_SYSTEM_FONT_FIXED:
+            uifont = kCTFontUIFontUserFixedPitch;
         default:
             break;
     }