configure.ac: set GLib 2.26 as minimal version
configure.ac: set GLib 2.26 as minimal version

libsocialweb claims to support GLib as old as 2.14, but there's no
macro for that; 2.26 is the oldest version for which a version macro
exists.

In particular, this gets rid of deprecation warnings about GValueArray.

Signed-off-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=695280
Reviewed-by: Ross Burton <4557ca660bca40225ba58749d212d65c650d01ed@burtonini.com>

GNOME/libsocialweb,GNOME/libsocialweb,GNOME/libsocialweb