[Vala] Preprocessor defines via Meson or valac

[Ken Gilmer <kgilmer gmail com>]

Hello!

I am trying to bind to libfuse from a Vala program.  I see that in libfuse
I must specify a define to the version of the API I want to use[4].  I read
the Vala documentation as to how to define a symbol[3].  I see that there
is documentation in meson for how to add arguments to valac[1] but when I
set my flag, add_project_arguments('-D FUSE_USE_VERSION=26', language:
'vala'), valac complains that it doesn't recognize the option and in the
build output panel in Gnome Builder it appears to wrap the expression in
single quotes.  When run valac directly from the command line and sidestep
meson, it seems that the define is ignored[2]. As a sanity test, if I
modify the header to define the value, I get the behavior I want.

My question is, how can I set a preprocessor symbol to a specific value via
Meson or valac?

TIA!
ken

1: https://mesonbuild.com/Vala.html <https://mesonbuild.com/Vala.html#>
2: valac -D FUSE_USE_VERSION=26 --pkg fuse --pkg gobject-2.0 --pkg glib-2.0
main.vala
3: https://wiki.gnome.org/Projects/Vala/Manual/Preprocessor
4:
https://github.com/libfuse/libfuse/blob/fuse_2_9_bugfix/include/fuse.h#L16