scripts: mk_util.py, support DESTDIR
scripts: mk_util.py, support DESTDIR

To be able to package z3 on distros, we need to install to a build
root. For that purpose, DESTDIR usually exists. So add this
functionality to the install script.

Signed-off-by: Jiri Slaby <dfbf39eb5bf303a3db5454d815cb8c8888cbadf9@suse.cz>

jirislaby/z3,jirislaby/z3,jirislaby/z3,jirislaby/z3,jirislaby/z3,jirislaby/z3