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.
Apparently configure was generating config.h, and nothing ever actually
included it. Include it in nasm.h as well as compiler.h, and in
rdoff/rdlar.h which uses these macros.
There appears to be no use of HAVE_* macros in any file that doesn't
have nasm.h included; in fact, so far the only use has been in
rdoff/rdlar.h.
Move anything compiler-specific to "compiler.h".
There was an unguarded use of __attribute__(()) in outmacho.c; also
require gcc 4+ for __builtin_ctlz(). Speed up the open-coded version, too.
