Makefile: Compile zlib .c files with DMD via importC

[kinke]

According to earlier CI results, this works - except for a problem on Win64 only:

phobos64.lib(gzwrite.obj) : error LNK2019: unresolved external symbol __va_start referenced in function gzprintf

So looks like this __va_start could be a candidate for inclusion in druntime's __builtins.di (which already defines a __builtin_va_start alias).

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Testing this PR locally

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[kinke]

Argh, the Microsoft headers use something like this (vadefs.h) for x86_64:

void __cdecl __va_start(va_list* , ...);
#define __crt_va_start_a(ap, x) ((void)(__va_start(&ap, x)))

The signature of their intrinsic isn't compatible with druntime's va_start (which takes the first param as out ref, not as explicit pointer), so I sadly don't see a way to handle this in druntime's __builtins.di (or importc.h).

I've worked around it for now, via #if __IMPORTC__ special case in the zlib source. :/

[dkorpel]

Is this ready to go?

[kinke]

The zlib hack obviously isn't ideal, but probably better than keeping on using the C compiler.

[dkorpel]

I opened an issue for it: https://issues.dlang.org/show_bug.cgi?id=24310

[ibuclaw]

Looks like this broke Windows builds on DMD.

https://github.com/dlang/dmd/actions/runs/7376357253/job/20069039989
https://github.com/dlang/dmd/actions/runs/7376357253/job/20069040050
https://github.com/dlang/dmd/actions/runs/7376357242/job/20069040244

../dmd/generated/windows/release/64/dmd.exe -c  -conf= -I../dmd/druntime/import  -w -de -preview=dip1000 -preview=dtorfields -preview=fieldwise -m64  -O -release -P=-Ietc/c/zlib -ofgenerated/windows/release/64/etc/c/zlib/compress.obj etc/c/zlib/compress.c
Error: Error: C preprocess command C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\bin\HostX64\x64\cl.exe failed for file etc\c\zlib\compress.c, exit status 2

C preprocess command C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\bin\HostX64\x64\cl.exe failed for file etc\c\zlib\adler32.c, exit status 2

etc\c\zlib\compress.c: fatal error C1083: Cannot open include file: '../dmd/druntime/import\importc.h': No such file or directory
make: *** [Makefile:322: generated/windows/release/64/etc/c/zlib/adler32.obj] Error 1
etc\c\zlib\adler32.c: fatal error C1083: Cannot open include file: '../dmd/druntime/import\importc.h': No such file or directory