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.

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@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).