bpo-4032: Cygwin: Add .dll.a to UnixCCompiler for searching libraries

[ma8ma]

Allow the UnixCCompiler.find_library_file to correctly find DLL import libs (.dll.a).

This is an alternative PR for #4136:

This simple patch allows UnixCCompiler.find_library_file, which searching library directories (e.g. /usr/lib), to recognize .dll.a as matches for the library. This is necessary on Cygwin where actual DLLs are not typically found under /lib, but rather /bin (because Windows searches PATH for DLLs). However one does .dll.a import libs under /lib (these are simple static archives that contain stubs for symbols actually found in the DLL, which can be used in lieu of the DLL itself for linking). This satisfies that the library being searched for is typically found.

This is especially necessary now that the _ctypes module can only be built with --with-system-ffi; without this patch the system libffi cannot be found.

The main difference to the original is the use of the implib_lib_extension variable. The original patch will overwrite the dylib_lib_extension variable which previously only had meaning on OSX, but is a bit of a misnomer for searching for DLL import libs on Cygwin.

https://bugs.python.org/issue4032

[embray]

Hi @ma8ma got pulled away to other things again, but I want to get back to this now. I have everything I need in place to run a CPython buildbot worker, we just need this PR and a few others merged so that it can work somewhat.

[ma8ma]

@embray Thank you for preparing the buildbot worker! Note that deadline of Python 3.7 features comes soon... so if you get ready for adding the worker, would you post on python-dev to request reviewing this PR?

[embray]

I will, though I've run into a problem getting the buildbot worker up and running, specifically: https://bugs.python.org/issue32243

Besides that there are a couple other issues (for which I have workarounds) that cause the test suite to hang. For the time being I could ask to add a Cygwin-specific build that is configured to skip those tests for the time being, but it's not ideal.

[ma8ma]

I see. Hmm, I'm not sure core developers will accept this change without a buildbot right now, therefore I won't get in present master (i.e. 3.7) branch while they aren't resolved...

[embray]

It would be great if we could get this merged. This is still a blocker to solving various other issues, because without this fix we can't build ctypes on Cygwin anymore.

[embray]

@ma8ma Could you rebase this PR? Or if not I'll open a new one based on it.

[ma8ma]

Hi @embray .
I'll do that!

[embray]

@ma8ma Awesome, thank you!

[embray]

PR #14013 is still dependent on this PR which has created some confusion. I think this should be an uncontroversial patch and just needs review. This is a patch that has always been needed for extension modules to compile properly on Cygwin.

[iritkatriel]

Closing the PR following the closure of its b.p.o issue (https://bugs.python.org/issue4032)