instead of relying on port's os.h to tell us. (Needed for HPUX
where system major version is not enough info.)
configure unsets USE_TK if X libraries not found.
doc/Makefile uses gzcat or zcat as found by autoconf.
Makefile.global and move them to seperate 'include' makefiles
Over time, should become even more port specific:
ie. Makefile.BSD44_derived should be broken down into netbsd/freebsd
specific ports