Add missing checks for gnulib vasnprintf()

Commit 212e4232 introduced a big update
of gnulib. Necessary changes to configure.ac from old gnulib commit
e8e63d1b31bca6c82713cba490b21a861abb24b5 have been forgotten. Actually
available functions are not discovered by autotools.

https://bugzilla.gnome.org/show_bug.cgi?id=759134