[RFC] Add platform checks to replace config.h.in hardcoded values

Basically cmake checks for some of the hardcoded defines in config.h.in

* Commented out a bunch of defines that are not being used (and neither are the functions they refer to)
* Not quite sure what to do about tgetent related checks, these should probably be handled when looking for curses/term
* Some of the more tricky checks are still missing, iconv(), st_blksize, langinfo
* In some cases I fail immediately for functions/headers that are implicitly assumed to be available (setenv(), lstat(), sys/wait.h)
I was planning to save this for another PR - specific to the curses bits in term.c - I think some of these can be removed. .e.g HAVE_TGETENT

HAVE_OSPEED will likely need c_source_compiles() (same for UP_BC_PC)