[build2] Issues with my first attempt at build2 CI

[Karen Arutyunov]

> Is there a specific reason you used file{} target instead of hxx{} ?
> These files are included so I assumed they should be considered as
> headers?

Probably you can consider them as headers but note that file{} targets 
just get distributed while hxx{} are also get installed by default.

After a bit of thinking I would probably write something like this:

lib{gtest}: src/cxx{gtest-all} include/hxx{**}
lib{gtest}: src/cxx{* -gtest-all -gtest_main}: include = false
lib{gtest}: src/hxx{*}: install = false

Best regards,
Karen