You can either try to install the GNU install program, and (if you did not put it in your global path) then point to the location with:

./configure --with-install=/path/and/name/of/install

Or you can use the default install delivered with IRIX 6.5 in which case you will have to edit the Makefile by hand a little bit. Change the following two lines:

  $(INSTALL) -c -m 644 DOCS/mplayer.1 $(MANDIR)/man1/mplayer.1

  $(INSTALL) -c -m 644 etc/codecs.conf $(CONFDIR)/codecs.conf

to:

  $(INSTALL) -m 644 mplayer.1 $(MANDIR)/man1/

  $(INSTALL) -m 644 codecs.conf $(CONFDIR)/

And then do (from within the MPlayer source dir):

cp DOCS/mplayer.1 . ; cp etc/codecs.conf .

and then go on with building and installing.