The eupspkg.cfg.sh file (in $LSSTSW/build/psfex/ups) allows a rebuild to incorrectly use whatever flavor of cc is found. The call to scons in build() should probably include cc="$CC", just like default_build() in $EUPS_DIR/lib/eupspkg.sh does.
This was discussed in the hipchat DM Newbies room on 2016-09-22. In my case, on macOS Sierra, psfex was attempting to build with homebrew's gcc instead of clang. This caused the build to fail.