execute_process: Add option to echo command lines

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

Help/command/execute_process.rst

@@ -18,6 +18,7 @@ Execute one or more child processes.
                   [ERROR_FILE <file>]
                   [OUTPUT_QUIET]
                   [ERROR_QUIET]
+                  [COMMAND_ECHO <where>]
                   [OUTPUT_STRIP_TRAILING_WHITESPACE]
                   [ERROR_STRIP_TRAILING_WHITESPACE]
                   [ENCODING <name>])
@@ -77,6 +78,10 @@ Options:
 ``OUTPUT_QUIET``, ``ERROR_QUIET``
  The standard output or standard error results will be quietly ignored.
 
+``COMMAND_ECHO <where>``
+ The command being run will be echo'ed to ``<where>`` with ``<where>``
+ being set to ``STDERR``|``STDOUT``|``NONE``.
+
 ``ENCODING <name>``
  On Windows, the encoding that is used to decode output from the process.
  Ignored on other platforms.