Need help with Windows linkage ( DMD using ImportC)

[Carl Sturtivant]

I found a way to make a solution for 64 bit Windows mechanically 
with many MSVC intrinsics, using only mingw64.
Here's an [MSYS2](https://www.msys2.org/) bash script.
```bash
gcc -E -P intrin.c -o vcintrinsics.c

sed -i 's/extern __inline__ 
__attribute__((__always_inline__,__gnu_inline__))//g' 
vcintrinsics.c

gcc -fPIC -shared -o vcintrinsics.dll vcintrinsics.c 
-Wl,--export-all-symbols -Wl,--output-def=vcintrinsics.def

#lib -nologo -machine:x64 -def:vcintrinsics.def 
-out:vcintrinsics.lib

dlltool -D vcintrinsics.dll -d vcintrinsics.def -l 
vcintrinsics.lib -m i386:x86-64

cp vcintrinsics.dll /c/D/dmd2/windows/bin64/
cp vcintrinsics.lib /c/D/dmd2/windows/lib64/
```
The commented out line is using the MS librarian to do the same 
job as the mingw64 libtool. This script builds a dll containing 
100+ intrinsics defined in mingw64 and makes an import library 
for DMD to use it from a developer command prompt.

`intrin.c` contains only `#include <intrin.h>` and is 
preprocessed in the first line into `vcintrinsics.c` which if 
examined contains working definitions of many intrinsics but with 
`extern __inline__ 
__attribute__((__always_inline__,__gnu_inline__))` prefixing 
them, so they will not compile to actual library functions. The 
sed command strips those prefixes out, and the compilation makes 
a DLL containing the the intrinsics and a DEF file listing their 
names for linkage, ready for dlltool to assemble an import 
library suitable for DMD to link to so as to be able to 
dynamically link the DLL.

And that's it; so far the result has just worked and I've had no 
linkage issues due to missing MSVC intrinsics since. I just put 
vcintrinsics.lib on the end of the dmd command line whenever I'm 
using real Windows headers in ImportC and the linker has always 
been satisfied so far. When I'm done, I can remove vcintrinsics 
from the dmd command line, and find out which intrinsics are 
actually needed for linking, and using the technique earlier in 
the thread above, satisfy those directly, so the result doesn't 
need vcintrinsics.dll.

Hopefully DMD will soon know about these MSVC intrinsics but 
until then this is an effective way to more-or-less permanently 
work around the inevitable linkage problems.