CMake overhaul

[derpda]

Sorry for the pull request out of the blue. I am suggesting major changes to the CMake scripts of yomm2. First let me explain why I was motivated to make these changes

Motivation

When using yomm2 as a dependency, we usually employ the same technique as suggested at for googletest: Download, build and locally install yomm2 automatically during CMake configure step using ExternalProject_Add and include the result in our main build. For yomm2, it was not possible to use CMake's find_package command to include yomm2 in our build since yomm2 does not define a proper CMake package upon installation. This is only a minor inconvenience, but was enough to motivate me to work on yomm2's CMake scipts. Additionally, "benchmark" was always downloaded even when YOMM2_ENABLE_BENCHMARKS was not set to ON, which takes unnecessary time.

Content of Pull Request

I have mainly made two changes:

1. Defined CMake package building instructions (install instructions) for yomm2
2. Overhaul of how the "benchmark" dependency is pulled and built

As for the first point, the commits are fairly short and are the best explanation of the content of my changes that I can come up with at the moment.

For the second point, there are a few advantages to the new method of automatically downloading and building "benchmark"

• We no longer use add_subdirectory, so a different CMAKE_BUILD_TYPE can be used (always RELEASE atm)
• We only download "benchmark" if YOMM2_ENABLE_BENCHMARKS=ON and it is not already installed
• Since "benchmark" is built during the configure stage, we can use find_package to include it (better encapsulation) The only disadvantage I can come up with is that we use execute_process to execute a second cmake during the configure stage. Command line settings are not automatically transferred to this nested call, but I cannot come up with a case where that would be a problem.

Conclusion

I believe the suggested changes make yomm2 a more complete library, as it is now a fully defined CMake package that can easily be used from user side code (like the library we're working on).

Please let me know if you have any questions, and I will try to respond quickly!

[derpda]

I forgot to add something: In the commit "Compilation of examples made optional" I have changed the script so that examples will only be built when YOMM2_BUILD_EXAMPLES=ON is set explicitly, just like for tests and benchmarks. I probably should have considered that you might want those to be built by default. If that is the case, we could set the default of YOMM2_BUILD_EXAMPLES to ON and instead allow their compilation to be turned off by setting YOMM2_BUILD_EXAMPLES=OFF. For user-side code like our library, this would be a nice setting to have since we don't need examples and they take much longer to build than the actual library since yomm2 is mostly headers.

[jll63]

Thank you so much for working on this! As you can tell, I did not spend a lot of time on the cmake stuff, which is not my field of expertise. Can you look take a look at this error please? https://travis-ci.org/github/jll63/yomm2/jobs/678822450

[jll63]

Also could you update the README.md? It still mentions the submodule. It would be nice if you could make it current with your changes.

[derpda]

There should be some way to make the CI output show up here for this pull request, that way I could confirm the results myself! I think I know how to fix the issue though.

[derpda]

Nevermind about setting up Travis CI for this project, looks like it's $69/month...

[jll63]

We're almost there: it passes on Travis but not on Windows. See my review. Thanks for the README :)

[derpda]

While doing some testing, I found an unrelated small issue in blackbox.cpp: Two tests have the same name and boost::tests complains about this, causing blackbox to fail.

[derpda]

This latest commit should fix the issue on Windows, could you confirm that for me?

[jll63]

This latest commit should fix the issue on Windows, could you confirm that for me?

It all looks good. Merging now. Thanks for this.