MinGW-w64 runtime library inclusion (Clang-compiled) for proper Win64 support?

[kink]

> I haven't checked what the exact licensing situation of the 
> MinGW runtime is (apart from some parts which I know are public 
> domain) but I imagine there might be potential problems in this 
> area.

Hi David, from what I've seen, the MinGW-w64 CRT seems to be 
entirely public domain.

I got the whole thing (latest v2.0.8) to compile; a few linking 
issues remain, working these out wouldn't be a problem. I chose 
to use the MinGW-w64 headers exclusively (no MS headers) to keep 
things simple and basically only had to adapt a few headers in a 
few places after I let Clang 3.4 run in a GCC-compatible way 
(cmdline options: -c -nostdinc -target x86_64-pc-win32 -Xclang 
-cxx-abi -Xclang microsoft -fno-ms-compatibility -fmsc-version=0).

The thing is that Clang maps C "long double" to double as well, 
just like MSVC. I'm not sure if that's generally the case when 
targeting Windows or only if Clang itself has been compiled with 
MSVC and not with MinGW. GCC's __float80 does not seem to be 
supported either. So I think there's probably a good reason for 
the dropped x87 support; I don't know about how well it is still 
supported in x86 hardware nowadays and how slow it is compared to 
scalar SSE code.

So maybe the best course of action would be to map D's real type 
to double as well, either as default setting in LDC for Windows 
or at least as option. I wouldn't miss the x87 type; I just want 
working reals support as it's used in Phobos' std.math all over 
the place.