[build2] Shell expansion in compiler arguments?

[David Cohoe]

Hello,

I am trying to use wxWidgets in a build2 project.  To compile anything
with it, you need to add the output of "wx-config --cxxflags" to your
compiler.  However, I can't seem to find a way to make build2 run that
command in the shell.  If I just put in $(wx-config --cxxflags) as an
option in the buildfile, it thinks I'm trying to use a variable, and
if I put it in single quotes, it gets passed directly to the compiler
without being called.  Is there any way to do this?  Of course I can
run the command myself and add the output manually, but the main
reason to do this is to make it portable, and I don't want to be
changing this for every new computer I use.