diff --git a/config/rules.mk b/config/rules.mk
index 1cd2e467e53..9241e99884e 100644
--- a/config/rules.mk
+++ b/config/rules.mk
@@ -1824,9 +1824,8 @@ libs::
 # be absolute (it's passed to tests for use in getting access to
 # files), we need to convert it.
 #
-# XXX find a better way to do this, because this probably isn't
-# guaranteed to work right everywhere
-_ABS_TOPSRCDIR := `dirname $(topsrcdir)`/`basename $(topsrcdir)`
+# XXX is this the right way to do it?
+_ABS_TOPSRCDIR := `realpath $(topsrcdir)`
 
 ifdef CYGWIN_WRAPPER
 NATIVE_TOPSRCDIR := `cygpath -wa $(_ABS_TOPSRCDIR)`