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.

Source/cmProcessOutput.cxx

@@ -13,7 +13,7 @@ cmProcessOutput::Encoding cmProcessOutput::FindEncoding(
   std::string const& name)
 {
   Encoding encoding = Auto;
-  if (name == "UTF8") {
+  if ((name == "UTF8") || (name == "UTF-8")) {
     encoding = UTF8;
   } else if (name == "NONE") {
     encoding = None;