Add support for DESTDIR

[mathstuf]

For packaging, DESTDIR is usually also required[1] as well as PREFIX. For Fedora guidelines, it would be required.

[1]http://www.gnu.org/prep/standards/html_node/DESTDIR.html

[geertj]

Note to self: i would like to move cgreenlet to autoconf/automake instead of the current bespoke Makefiles.