cmd/gc: fix windows build, probably

TBR=iant
CC=golang-dev
https://golang.org/cl/39540043

src/cmd/gc/go.h

@@ -1182,7 +1182,6 @@ char*	expstring(void);
 void	mkpackage(char* pkgname);
 void	unimportfile(void);
 int32	yylex(void);
-extern	int	windows;
 extern	int	yylast;
 extern	int	yyprev;
 

src/cmd/gc/lex.c

@@ -14,7 +14,6 @@
 #define	ungetc	ccungetc
 
 extern int yychar;
-int windows;
 int yyprev;
 int yylast;
 
@@ -548,7 +547,7 @@ islocalname(Strlit *name)
 {
 	if(name->len >= 1 && name->s[0] == '/')
 		return 1;
-	if(windows && name->len >= 3 &&
+	if(ctxt->windows && name->len >= 3 &&
 	   yy_isalpha(name->s[0]) && name->s[1] == ':' && name->s[2] == '/')
 	   	return 1;
 	if(name->len >= 2 && strncmp(name->s, "./", 2) == 0)
@@ -2342,7 +2341,7 @@ mkpackage(char* pkgname)
 
 	if(outfile == nil) {
 		p = strrchr(infile, '/');
-		if(windows) {
+		if(ctxt->windows) {
 			q = strrchr(infile, '\\');
 			if(q > p)
 				p = q;