This removes the check for "mingw" for platform.system(). The only case I know
where "mingw" is return is if using a msys Python under a msys2 mingw environment.
This combination is not really supported by meson and will result in weird errors,
so remove the check.
The second change is checking sys.platform for cygwin instead of platform.system().
The former is document to return "cygwin", while the latter is not and just
returns uname().
While under Cygwin it uname() always starts with "cygwin" it's not hardcoded in MSYS2
and starts with the environment name. Using sys.platform is safer here.
Fixes#7552
D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082
t.pic won't be defined. We can only hope it has been built with -fPIC.
Linker will complain otherwise any way.
t.extract_all_objects_recurse() won't be defined. We could support this
case by extracting the archive somewhere and pick object files.