This allows us to disable the shared library test if the GNU D compiler
is detected.
This is useful, because gdc can not yet build proper shared libraries.
We also add a new test here which tests versioning of D shared
libraries.
D allows programmers to define their tests alongside the actual code in
a unittest scope[1].
When compiled with a special flag, the compiler will build a binary
containing the tests instead of the actual application.
This is a strightforward and easy way to run tests and works well with
Mesons test() command.
Since using just one flag name to enable unittest mode would be too
boring, compiler developers invented multiple ones.
Adding this helper method makes it easy for people writing Meson build
descriptions for D projects to enable unittestmode.
[1]: https://dlang.org/spec/unittest.html
Mike Wey
Matthias Klumpp
Nirbheek Chauhan