cgo: interpret $CGOPKGDIR as absolute path if rooted

R=dho
CC=golang-dev
https://golang.org/cl/180099
diff --git a/src/Make.pkg b/src/Make.pkg
index 890f43d..b315b2e 100644
--- a/src/Make.pkg
+++ b/src/Make.pkg
@@ -23,7 +23,11 @@
 TARG_words=$(subst /, ,$(TARG))
 elem=$(word $(words $(TARG_words)),$(TARG_words))
 
-dir=$(patsubst %/$(elem),%,./$(TARG))
+ifeq ($(elem),$(TARG))
+dir=
+else
+dir=$(patsubst %/$(elem),%,$(TARG))
+endif
 
 # ugly hack to deal with whitespaces in $GOROOT
 nullstring :=