Xcode: Invert quoting logic to whitelist of characters

Our explicit list of characters that require quoting does not account
for non-ascii characters (e.g. UTF-8) that also need quoting.  Invert
the logic to quote a string unless it contains exclusively a set of
characters we whitelist.

Source/cmXCodeObject.cxx

@@ -243,7 +243,11 @@ void cmXCodeObject::PrintString(std::ostream& os,std::string String)
   bool needQuote =
     (String.empty() ||
      String.find("//") != String.npos ||
-     String.find_first_of(" <>+-*=@[](){},~") != String.npos);
+     String.find_first_not_of(
+       "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+       "abcdefghijklmnopqrstuvwxyz"
+       "0123456789"
+       "$_./") != String.npos);
   const char* quote = needQuote? "\"" : "";
 
   // Print the string, quoted and escaped as necessary.