Change the code to store D properties as plain data. Only convert them
to compiler flags in the backend. This also means we can fully parse D
arguments without needing to know the compiler being used.
Older versions of GDC had broken support for the -MQ flag (resulting in
a compiler error). New versions of GDC have this issue fixed (some older
versions received a backport). Starting with GCC 7.1 we can assume
dependency generation to always work reliably.
Of course D compilers have different flags to set some important
D-specific settings. This adds a simple method to change these flags in
a compiler-agnostic way in Meson.
This replaces the previous `unittest_args` method with a more generic
variant.
Jussi Pakkanen