[BUG] Hardcoded python and install on solaris

Hi;

When compiling git-1.8.0.2 on a moderately old OpenIndiana machine, I had
to install a few things (m4, autoconf, coreutils, xz, python).

Even though I started the configuration fresh (make distclean; configure),
the makefile still wanted to use /usr/bin/python (instead of
/usr/local/bin/python) and /usr/usb/install (instead of
/usr/local/bin/install).

Thanks for a great SCM!