templates/Makefile: install is unnecessary, just use mkdir -p

templates/Makefile: install is unnecessary, just use mkdir -p

The native install on some platforms (namely IRIX 6.5) treats non-absolute
paths as being relative to the root directory rather than relative to
the current directory. Work around this by avoiding install in this case
since it is unnecessary, and instead depend on the local umask setting
and use mkdir.

Tested-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>