add buildsdk: tool to create COFF import libraries from MinGW def files

[rainers]

add tool to build a simple replacement for the VC runtime and the Windows SDK using the preinstalled MSVCRT.DLL and import libraries generated from MinGW definition files.

Compilation will have to be done with -mscrtlib=msvcrt51 to pick up this runtime.

To be integrated with the installer builds...

[dlang-bot]

Thanks for your pull request, @rainers!

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[rainers]

Building slightly more elaborate programs (e.g. using writeln) revealed that I've been a bit too optimistic here: phobos needs symbols that are not available in MSVCRT.DLL, but need MSCR800.DLL or later.

That means we might need to install a VC redistributable package as well as mimick OLDNAMES.LIB for C symbols no longer in the DLL.

[dnadlinger]

Do I understand correctly that distributing the resulting files (modulo the DLL availability issue) would allow linking without MinGW or Windows SDK being present (with LDC's -link-internally then being truly stand-alone)?

[rainers]

would allow linking without MinGW or Windows SDK being present

Yes, it also allows to dynamically link against the VS2010 C runtime from the redistributables (other versions should be easy to adapt to, but MinGW support seems to be limited to VS2013). The idea is to bundle this with lld-link for a standalone installation without further dependencies.

LDC is a bit different as it only supports VS2015 or later and requires the C++ runtime (for exceptions). That would need additional work.

[rainers]

Closing in favour of adding this to the installer repo: dlang/installer#289