ExternalProject: Avoid adding empty touch command on "ALWAYS" steps

Antonio Caggianorequested to merge
fahien/cmake:fahien/fix-touch into master

The problem fixed by commit c0b749cf may occur for an empty COMMAND ${touch} too, so avoid specifying any COMMAND at all if there is no touch command.

Topic-rename: ExternalProject-touch-echo

Edited by Brad King

Merge request reports