[Vala] Why valac is always called even if conditional automake clause is false?

["marcin saepia net" <marcin saepia net>]

Hello,

I have the following makefile that is responsible for conditional
compilation of one subdir of my project depending on the platform used:
https://gist.github.com/mspanc/5b31758b966fa6eab959

I am using automake's AM_CONDITIONAL to pass information about operating
system to further scripts.

It works (almost) well, the code compiles, but I've noticed that when I
call make in this subdir, it calls valac for all source files, even these
not matched by current condition:

$ make
rm -f libwhatever_common_media_audio_la_vala.stamp && echo stamp >
libwhatever_common_media_audio_la_vala.stamp-t
CDPATH="${ZSH_VERSION+.}:" && cd . && /usr/local/bin/valac --library
libwhatever-common-1.0-media-audio --vapi
libwhatever-common-1.0-media-audio.vapi -H media-audio.h
--enable-experimental --enable-experimental-non-null  -D PLATFORM_DARWIN
-C lifecycle.vala base_interface_monitor.vala
base_polling_interface_monitor.vala interface_monitor.vala
windows/lifecycle.vapi windows/interface_list.vapi
windows/interface_monitor.vala darwin/interface_list.vapi
darwin/interface_monitor.vala linux/interface_monitor.vala

(here should exclude windows/ and linux/ subdir as it was called on mac)

VALAFLAGS are set correctly, so conditionals are working fine for other
cases.

How to prevent this?

Is it a bug in automake's vala extensions or am I doing something wrong?

Thanks,

Marcin