build: add an additional check for glib-mkenums

build: add an additional check for glib-mkenums

Some distros split glib-mkenums and other GLib build utilities into a
separate package but the glib-2.0 pkgconfig file pointing to it is
typically shipped as part of the -dev package. This can lead to a
situation where configure thinks it knows where glib-mkenums is but make
then fails because it's not really installed on the system. Check the
existence of the actual executable in addition to querying pkgconfig.

Link: https://lore.kernel.org/r/20241004092533.34368-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>