Error message in full:

[[email protected] kernel]$make xconfig
  HOSTCXX scripts/kconfig/qconf.o
In file included from scripts/kconfig/
scripts/kconfig/qconf.moc:13:2: error: #error "This file was generated using the moc from 4.6.3. It"
scripts/kconfig/qconf.moc:14:2: error: #error "cannot be used with the include files from this version of Qt."
scripts/kconfig/qconf.moc:15:2: error: #error "(The moc has changed too much.)"
make[1]: *** [scripts/kconfig/qconf.o] Error 1
make: *** [xconfig] Error 2

A make clean followed by repeating it did not help.

Finally, I just deleted the offending file.

[[email protected] kernel]$mv scripts/kconfig/qconf.moc ./
[[email protected] kernel]$make xconfig
/usr/bin/moc -i scripts/kconfig/qconf.h -o scripts/kconfig/qconf.moc
  HOSTCXX scripts/kconfig/qconf.o
  HOSTLD  scripts/kconfig/qconf
scripts/kconfig/qconf Kconfig

It then worked.

Thanks to Goverp from Gentoo forum for the tip.