custom build command for any *.myextension file

[pako]

I'm using codeblocks for my fltk-based project.
And I want my IDE to run "fluid -c $file" for any *.fl file in my project when i'm building that project.

It is possible to add custom build command for each .fl file in the project, but I think there must be a better way.

There is a "custom variable" field in the file properties, above the "custom build command" field.
Its default value is CPP. What does that "custom variable" do?