configure.ac: add option "--disable-glib"

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

src/system/FatalError.hxx

@@ -45,9 +45,11 @@ gcc_noreturn
 void
 FatalError(const char *msg, const Error &error);
 
+#ifdef HAVE_GLIB
 gcc_noreturn
 void
 FatalError(const char *msg, GError *error);
+#endif
 
 /**
  * Call this after a system call has failed that is not supposed to