pre#defines for system and machine

[gwyn at brl-smoke.UUCP]

The point about using lower-level "feature defines" like #ifdef TCGETA
is well taken, but there is also use for higher-level non-feature
conditionalization.  For example, my most common conditional seems to
be #if BRL_System_V_emulation_really_running_on_a_4BSD_kernel
(or equivalent).  I know of no reliable way to test combinations of
features to determine high-level properties like that.

I could also really use #if_header_exists "header_name" but I doubt we
could come up with a good way to wedge this into C portably.  However,
there might be hope; a small subgroup of X3J11 is working on defining
minimum syntax for portable filenames for use in #include "header_name"
directives.  Several of us feel that that is needed to plug a major hole
in the facilities a maximally-portable C application can rely on.