david hoskin found a bug in cpp.  - is evaluated right to left, so
it computes the wrong result.  for example

	# if 0 - 0 + 4 != 4
	# error bogus preprocessor
	# endif

errors, because cpp computes this as if it were

	# if 0 - (0 + 4) != 4

this is because it evaluates right to left, not right to left.  it turns
out this is because the evalop uses < rather than <= as the
pop (loop) condition for the prec stack.  thus a + b + c
will push to the end then pop from the right.

while the according-to-hoyle fix is to change < to <=, this was
done on purpose.  it allows symbols that need no eval to be
omitted.  it would be a major rework to change this assumption.

so the hack-upon-hack solution might be to just convert - to
+ UMINUS(thing).  this is going to cause trouble if someone
is doing fancy arithmetic with division and counting on
non-underflow.

Reference: /n/atom/patch/applied/cppifltorhack
Date: Sun Mar  2 17:51:24 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/cpp/eval.c	Sun Mar  2 17:51:10 2014
+++ /sys/src/cmd/cpp/eval.c	Sun Mar  2 17:51:11 2014
@@ -122,6 +122,7 @@
 	op = ops;
 	*op++ = END;
 	for (rand=0, tp = trp->bp+ntok; tp < trp->lp; tp++) {
+	retry:
 		if(op >= ops + NSTAK)
 			sysfatal("cpp: can't evaluate #if: increase NSTAK");
 		switch(tp->type) {
@@ -161,6 +162,12 @@
 				}
 				continue;
 			}
+			if(tp->type==MINUS){
+				*op++ = UMINUS;
+				tp->type = PLUS;
+				goto retry;
+			}
+				
 			/* flow through */
 
 		/* plain binary */