FRONTEND: check for glibc ver and correct gtk flag

[Apaczer]

Otherwise it will fail build on glibc prev. to 2.74, which may be trouebling while cross-compiling for older architectures:

src/frontends/notify.c:64:77: error: ‘G_APPLICATION_DEFAULT_FLAGS’ undeclared (first use in this function); did you mean ‘G_APPLICATION_GET_CLASS’?
   64 |                         application = g_application_new("gmu.music.player", G_APPLICATION_DEFAULT_FLAGS);
      |                                                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                                             G_APPLICATION_GET_CLASS
src/frontends/notify.c:64:77: note: each undeclared identifier is reported only once for each function it appears in
make: *** [Makefile:237: frontends/notify.so] Error 1

for reference: https://docs.gtk.org/gio/flags.ApplicationFlags.html#flags_none

[Apaczer]

didn't bother to look at repo's git log, but corrected myself!