build: allow mingw VPATH build

* .gnulib: Update to latest.
Reported by Matthias Bolte.

diff --git a/.gnulib b/.gnulib
index 1629006348e1f66f07ce3ddcf3ebd2d14556cfce..7ba06c8ff37d8725cb824c64b94be41b9294ffb7 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit 1629006348e1f66f07ce3ddcf3ebd2d14556cfce
+Subproject commit 7ba06c8ff37d8725cb824c64b94be41b9294ffb7