Fix unit test when char is unsigned by default

Skip test relying on char being signed if it isn't by default, as is the case
with gcc on some ARM platforms, for example.

Closes https://github.com/wxWidgets/wxWidgets/pull/299

tests/strings/vararg.cpp

@@ -139,8 +139,10 @@ void VarArgTestCase::CharPrintf()
     #ifdef _MSC_VER
         #pragma warning(default:4309)
     #endif
+    #ifndef __CHAR_UNSIGNED__
     s.Printf("value is %i (int)", c);
     CPPUNIT_ASSERT_EQUAL( wxString("value is -16 (int)"), s );
+    #endif
 
     unsigned char u = 240;
     s.Printf("value is %i (int)", u);