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: NEric Blake <eblake@redhat.com>

.gnulib @ 246b3b28

-Subproject commit 8d807a99c6e8eecd2a9cf7c7b5d48ec0b2c934f8
+Subproject commit 246b3b28808ee5f4664be674dce573af9497fc7a