compile .c files with ImportC

[WalterBright]

This is a work in progress to use ImportC to compile all the .c files in Phobos.

[dlang-bot]

Thanks for your pull request, @WalterBright!

Bugzilla references

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

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 "master + phobos#8484"

[WalterBright]

Hmm, different errors on every platform :-/

[WalterBright]

This is yet another test run where the only error message is:

osix.mak:413: recipe for target 'unittest/std/zlib.run' failed
make[1]: *** [unittest/std/zlib.run] Error 139

and a thousand lines of where or what failed gives no help at all.

[LightBender]

We'll do this a different way with Phobos 3.