build: Bump GLib minimum required to 2.26
This is the minimum required to build since xfce4-dev-tools >= 4.17.1
set GLIB_VERSION_MAX_ALLOWED
to the GLib version found in
configure.ac.in
.
@alexxcons maybe you want to take a look at this? I'm doing a round up of the GLib requirements following apps/xfburn#54 (closed), which revealed that xfce4-dev-tools >= 4.17.1 can cause some problems if you're not careful (see apps/xfburn#54 (comment 60687) and bottom of page where there is also a list of related MRs).
Here the build simply fails, because GLIB_VERSION_2_18
that xfce4-dev-tools wants to use does not exist (these macros only exist since 2.26).