When running "?c -p ...", ensure the backend cpp recognizes C++ comments.

2c(1) states that the compilers recognize // comments, and the bare compilers
do.  But if you invoke the compiler with '-p', the backend cpp process
doesn't handle // comments properly unless you also give ?c the undocumented
'-+' option (which it passes through to cpp).

Reference: /n/sources/patch/applied/cc-cpp-c99-comm
Date: Mon Oct 31 22:34:22 CET 2011
Signed-off-by: lyndon@orthanc.ca

--- /sys/src/cmd/cc/lex.c	Mon Oct 31 22:29:37 2011
+++ /sys/src/cmd/cc/lex.c	Mon Oct 31 22:29:34 2011
@@ -242,12 +242,10 @@
 			close(fd[1]);
 			av[0] = CPP;
 			i = 1;
+			sprint(opt, "-+");
+			av[i++] = strdup(opt);
 			if(debug['.']){
 				sprint(opt, "-.");
-				av[i++] = strdup(opt);
-			}
-			if(debug['+']) {
-				sprint(opt, "-+");
 				av[i++] = strdup(opt);
 			}
 			for(c = 0; c < ndef; c++) {