maint: update to latest gnulib

Fix a regression in checking for realpath (which caused link
failures regarding duplicate rpl_canonicalize_file_name), and
fix the mingw build regarding unsetenv.

* .gnulib: Update to latest.

Signed-off-by: Eric Blake <eblake@redhat.com>

diff --git a/.gnulib b/.gnulib
index 8d807a99c6e8eecd2a9cf7c7b5d48ec0b2c934f8..246b3b28808ee5f4664be674dce573af9497fc7a 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit 8d807a99c6e8eecd2a9cf7c7b5d48ec0b2c934f8
+Subproject commit 246b3b28808ee5f4664be674dce573af9497fc7a