Concentrate compiler dependencies to compiler.h; make sure compiler.h
is included first in every .c file (since some prototypes may depend
on the presence of feature request macros.)
Actually use the conditional inclusion of various functions (totally
broken in previous releases.)
To deal with fools^Wpeople trying to keep really old systems alive,
create a proper framework for substitution functions, and make it
possible to deal with the lack of snprintf/vsnprintf in particular.
