Fix configure scripts to properly ignore absence of pilot-link library

Try to ignore absence of pilot-link library when 'ruby configure.rb' is called without  --disable-palm option.

Also, mkmf emits an error when no Makefile exists in the working directory upon exit, so don't require 'mkmf' when pilot-link is not installed or disabled and create_makefile is not called (You don't see the error when old Makefile exists).

By the way, even when I installed pilot-link (0.12.5), I couldn't build mhc_pilib.c on my FreeBSD machine.  Maybe palm support should be simply dropped?