The masked mod_caucho-3.0.19 ebuild fails to compile when a -j option is set in MAKEOPTS in make.conf (For me, it was MAKEOPTS="-j3"). I didn't save the error, but it couldn't find config.o, even though it appeared to have compiled and when I looked at the work directory it exited...so I removed the MAKEOPTS -j flag and it compiled correctly without problems. This is true on both the x86 and amd64 platforms I run, both of which have SMP to utilize the -j.
Now the ebuild forces -j1.