cpp: support linking statically on Windows

[wrightsg]

Changelog

Allows building and linking the mcap library statically on Windows.

Docs

None

Description

I'm using mcap_builder to use the mcap library in my project.

Even though mcap is a header-only library, mcap_builder creates a library for the client application to link against.
mcap_builder currently always creates a shared library. I have a PR open to support creating a static library: olympus-robotics/mcap_builder#4

Creating and linking against a static library on Windows causes linking issues because visibility.hpp implicitly assumes dynamic linking.

This PR adds a define that needs to be set when building the library statically. The PR is backwards compatible, i.e. the behavior remains the same as before if the define is not set. The define is named based on CMake convention, see https://cmake.org/cmake/help/latest/module/GenerateExportHeader.html

[defunctzombie]

A few thoughts after a quick review:

1. I don't like the undocumented and un-commented MCAP_STATIC_DEFINE
2. This seems like an issue created by mcap_builder and not specific to the mcap library here.

What is an alternative approach that does not mean introducing CMAKE specific un-documented defines to this library?

[wrightsg]

I guess this could be solved in mcap_builder by not compiling the mcap library into a static or dynamic library but to consume it as header-only library (through CMake interface library).

However, mcap already provides the MCAP_PUBLIC #define which is only required when compiling the mcap library into a static or dynamic library. What is missing, is the support for the specific combination of compiling it as a static library on Windows. The other combinations (dynamic/Windows, static/Linux, dynamic/Linux) are already supported. And not supporting one specific combination is inconsistent.

Regarding documentation, I'm happy to add this to the PR if you point me to the location where it should go (comment in the visibility.hpp header or in some README?).

[james-rms]

I think this is fine so long as this additional flag is explained in cpp/README.md under the "Including in your Project" heading.

[james-rms]

Will merge after readme is updated.

[defunctzombie]

@wrightsg ping on updating the readme and then we can merge