With the bindists (not sure about a normal build tree) install-strip doesn't work:

$ make install-strip
make: *** No rule to make target `install-strip'.  Stop.
$

It is defined in mk/install.mk, so it presumably is meant to. The blurb after running configure should mention it, too.

The target is described in the GNU coding standards: http://www.gnu.org/prep/standards/html_node/Standard-Targets.html