Makefile: Compile zlib .c files with DMD via importC

[kinke]

Depends on #8862.

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).

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "makefiles + phobos#8863"