Add support for libtool over ar

[keith]

On macOS libtool is preferred for creating static archives over ar. If
libtool is set as the static linker, we need to pass different flags.

Fixes bazelbuild/bazel#9258

[meteorcloudy]

@irengrig Can you review this? It's need to make downstream green again. ;)

[buchgr]

Irina is out of office for the next two weeks.

[meteorcloudy]

@hlopko @oquenchil Does this PR looks safe to you? We need some CMake/C++ expert.

[irengrig]

Hi, sorry for the late response, I am having a vacation (and turned off the notifications).
Thank you so much for working on this problem and submitting the pull request!

The documentation for the used flags is here: https://cmake.org/cmake/help/v3.15/variable/CMAKE_LANG_ARCHIVE_CREATE.html?highlight=cmake_%20lang%20_archive_create
As you can see, there are also 2 more related flags, for appending to the static archive, and finishing the process.
I guess we could set those into empty values, if we believe that <>_CREATE flag is enough.

The code looks reasonable. The reference to libtool cli: https://www.gnu.org/software/libtool/manual/html_node/Link-mode.html#Link-mode
However, I am not sure about <LINK_FLAGS>, whether the value will be compatible.

Also, the CI has passed, but it runs against released Bazel, not against the master branch.
It would be cool if @meteorcloudy could check that the fix indeed helps, and then merge this PR.

Thank you all again!

[keith]

Hmm I guess it's up to the cmake configs to know if those other flags are necessary. I can try to add the appropriate ones for those (although I'm not sure libtool supports those).

Good point about LINK_FLAGS. Maybe we should remove those. Alternatively do you know if there's a way to make cmake smarter about the static archiver it's using? AFAICT there is not.

I think we should try this with bazel HEAD against envoy since it uses a ton of cmake libraries.

[irengrig]

Hmm I guess it's up to the cmake configs to know if those other flags are necessary. I can try to add the appropriate ones for those (although I'm not sure libtool supports those).

Well, I can google :) I think I rather add a test in a separate PR that the user can still pass the value for those flags to override the value that we put automatically, so that there is a workaround for any situation.

Good point about LINK_FLAGS. Maybe we should remove those. Alternatively do you know if there's a way to make cmake smarter about the static archiver it's using? AFAICT there is not.

Probably, each library can have this in CMakeLists.txt, but not outside the scripts. We can only manipulate the flags/environment variables.

I think we should try this with bazel HEAD against envoy since it uses a ton of cmake libraries.
Good idea!

[meteorcloudy]

I tested locally, this change doesn't look like to fix the issue. You can get a Bazel binary from HEAD at the following url to reproduce.

https://storage.googleapis.com/bazel-builds/artifacts/macos/20b056ae8506e489933cf236b9401cddad90afac/bazel

[keith]

#315

[irengrig]

@keith thank you again for working on it! I used the changes in this OR as the basis for the fix in #315.

Your fix was actually correct, the problem was in on eof the rules_foreign_cc tests with configure rule.
However, I changed the fix to be "less hard coded", to get the command line for linking from Bazel's cc_common interface.
That is the reason I submitted my own PR, based on your fix.