cmd/pack: support removing of leading file prefix on windows

Fixes #5550

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/11904043

src/cmd/pack/ar.c

@@ -1618,6 +1618,25 @@ int (*reader[256])(Biobuf*, Prog*) = {
 	[Obj386] = _read8,
 };
 
+#define isdelim(c) ((c) == '/' || (c) == '\\')
+
+/*
+ *	check if p is start of windows full path, like C:\ or c:/.
+ *	return 1 if so. also set drive parameter to its
+ *	upper-case drive letter.
+ */
+int
+iswinpathstart(char *p, char *drive)
+{
+	if('A' <= p[0] || p[0] <= 'Z')
+		*drive = p[0];
+	else if('a' <= p[0] || p[0] <= 'z')
+		*drive = p[0] - ('a' - 'A');
+	else
+		return 0;
+	return p[1] == ':' && isdelim(p[2]);
+}
+
 /*
  *	copy b into bp->member but rewrite object
  *	during copy to drop prefix from all file names.
@@ -1630,7 +1649,7 @@ arread_cutprefix(Biobuf *b, Armember *bp)
 	vlong offset, o, end;
 	int n, t;
 	int (*rd)(Biobuf*, Prog*);
-	char *w, *inprefix;
+	char *w, *inprefix, d1, d2;
 	Prog p;
 	
 	offset = Boffset(b);
@@ -1666,12 +1685,15 @@ arread_cutprefix(Biobuf *b, Armember *bp)
 			if(inprefix == nil && prefix[0] == '/' && p.id[1] == '/' && p.id[2] == '\0') {
 				// leading /
 				inprefix = prefix+1;
+			} else if(inprefix == nil && iswinpathstart(prefix, &d1) && iswinpathstart(p.id + 1, &d2) && d1 == d2 && p.id[4] == '\0') {
+				// leading c:\ ...
+				inprefix = prefix+3;
 			} else if(inprefix != nil) {
 				// handle subsequent elements
 				n = strlen(p.id+1);
-				if(strncmp(p.id+1, inprefix, n) == 0 && (inprefix[n] == '/' || inprefix[n] == '\0')) {
+				if(strncmp(p.id+1, inprefix, n) == 0 && (isdelim(inprefix[n]) || inprefix[n] == '\0')) {
 					inprefix += n;
-					if(inprefix[0] == '/')
+					if(isdelim(inprefix[0]))
 						inprefix++;
 				}
 			}