Fixed with this patch:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20061204/041149.html

The patch doesn't really fix the bug. It defines DESTDIR in the
makefile, something the GNU standards I linked to on opening the bug
explicitly rule out: "You should not set the value of DESTDIR in your
Makefile at all".

And there still isn't a variable called `prefix' that can be overridden
on the command line.

So the patch has implemented its own non-standard scheme which will just
cause confusion in the long run for packagers. May as well get it right
now, if at all.

PR739.patch
This is a patch I cooked up. It provides a bare-minimum of GNU "compatability",
at the make command level. "prefix=/some/dir" can be passed in, and overrides
the ./configure prefix, and "DESTDIR=/some/dir" can be passed in to install
into a staging directory other than root. This is (from my experience and
reading the posts above) the intended purpose for both of these arguments. No
other GNU make options have been emulated in this way to keep the makefiles
clean, as this is not a GNU project.

Also, I re-arranged the makefile slightly to allow both projects and the main
LLVM build to share the same functionality without duplicating all of the
directory definitions. If it is actually desirable to have directories defined
in a different manner from one to the other (other than obviously using
separate prefixes, as is already done), I can take this out.

I hope this addresses all the concerns for this bug, I know that my building of
Gentoo ebuilds for LLVM will be eased by just this level of functionality.

This is now mine.

Tested. Works. Patch:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070115/042849.html

Resolved.