Add COMMAND_ECHO option to the execute_process command. This will allow
execute_process to show the command it will run. Also add a cmake variable
CMAKE_EXECUTE_PROCESS_COMMAND_ECHO. Both the option and the variable can
be set to one of the following: STDERR|STDOUT|NONE. The command will be
printed to stderr or stdout or not at all.
Fixes: #18933
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.
Use the KWSys Process "MergeOutput" option to give the child process
the same pipe (or file) for both stdout and stderr. This allows
natural merging of stdout and stderr together instead of merging
on arbitrary buffered read boundaries as before.
