execute_process: Allow UTF-8 as a synonym for the UTF8 keyword

UTF-8 is the proper naming according to the UTF-8 RFC and is also the
name used for a similar keyword in the file() command. This commit
brings (backward compatible) consistency to the keyword names and allows
the standard UTF-8 name to be used with execute_process(). The old UTF8
keyword is still supported.

Tests/CMakeLib/testEncoding.cxx

@@ -31,7 +31,7 @@ int main(int argc, char* argv[])
   }
   const std::string encoding(argv[1]);
 #ifdef _WIN32
-  if (encoding == "UTF8") {
+  if ((encoding == "UTF8") || (encoding == "UTF-8")) {
     setEncoding(consoleOut, CP_UTF8);
   } else if (encoding == "ANSI") {
     setEncoding(consoleOut, CP_ACP);