share/termcap requires ex from usr.bin/vi in order to compile, and since usr.bin
comes after share in SUBDIR in Makefile.inc1, the build will fail when vi is not
installed on the build host

Run build-tools for usr.bin/vi and install ex, etc to WORLDTMP to enable building
share/termcap on hosts that don't have nvi installed on them
Fill in MK_VI == no
Implement mdoc cross references
Make ee(1) optional via MK_EE