build: Bump GLib minimum required to 2.38
This is required by G_DEFINE_TYPE_WITH_PRIVATE(), and xfce4-dev-tools >= 4.17.1 set GLIB_VERSION_MAX_ALLOWED to the GLib version found in configure.ac.in. This usually only generates deprecation warnings, but in this case it changes the code path used at compile time and causes memory corruption and a crash.
Related: xfburn#54 (closed)