The emerge man page should list all the environment options it indirectly supports, like PORTAGE_TMPDIR etc. This particular one is mentioned in the install documentation, but who remembers that. :-) And re-emerging X takes so many extra megabytes of temp space that half the people trying would run out. :-)
Glad to do it. I'm going to talk to Daniel and find out what environment variables there actually /are,/ so it can be a more complete document.
drobbins has been very busy lately. Rest assured, however, that I'm gently bugging him until I get this information so I can document it.
check out the latest manpage http://cvs.gentoo.org/cgi-bin/viewcvs.cgi/gentoo-src/portage/man/make.conf.5?rev=1.5&content-type=text/vnd.viewcvs-markup