[Development] Getting build flags for platforms without pkg-config

[Jeandet Alexis]

Hello,

Previously I asked about getting some defines from pkg-config, I pushed
some code on gerrit to fix that.

Now I would like to care about platforms where we don't use pkg-config, 
at first I thought that we could get compile flags from qmake but it
seems that no.
Did I miss something?
Would there be a way to get compile flags for a build system like
meson?

Best regards,
Alexis.