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.