Did some debugging and the only thing I changed was switched my gcc back down to 4.2.3 and both m4's compiled fine. This doesn't really seem like a prefix bug, imo. build.log coming soon.
% eix -ec m4 [I] sys-devel/m4 (1.4.10-r2@03/31/2008): GNU macro processor % gcc-config -l [1] i686-pc-linux-gnu-4.2.3 * [2] i686-pc-linux-gnu-4.3.0
Created attachment 147858 [details] build.log with gcc-4.3
Dang, this bug is INVALID. Upon further digging, I see that gcc-4.3 has NO keywords in gentoo-x86 tree. How did it get keywords in the prefix tree? Simple fix then. Remove gcc-4.3's keywords and wait until upstream (gentoo-x86) fixes m4..?
I think this is a little problem I introduced. I guess we should mask it in prefix.
I masked gcc-4.3.0 for now.