Add MAKE_INSTALL_OPTS for packages

Allows for passing environment variables specifically to the "make install"
step of package building.

This can be helpful for providing a different DESTDIR= to a ruby install from
the --prefix that is used
Add tests for RUBY_MAKE_INSTALL_OPTS=
Add mention of RUBY_MAKE_INSTALL_OPTS in README
Add MAKE_INSTALL_OPTS configuration option
Add test for MAKE_INSTALL_OPTS

Also fix the ordering of make install arguments
Merge pull request #493 from eric/add-make-install-opts-variables

Add MAKE_INSTALL_OPTS for packages