build: Calculate ABS_GLIB_RUNTIME_LIBDIR at build time

Rather than calculating it at configure time. This means it can expand
$libdir properly, and use the Make $(realpath) function rather than
invoking the non-portable `readlink -f`.

This fixes problems where `readlink` would be called on an invalid path
(due to a variable not being expanded) and would evaluate to "", which
would then cause things to be installed in the wrong place.

https://bugzilla.gnome.org/show_bug.cgi?id=744772