Add a CMake option to not build the tests

[hedtke]

I created a Conan package for yoga, but I have to patch out the line add_subdirectory(tests) in CMakeLists.txt because I do not want to build the tests when creating the package. A CMake option would be nice, something like option(BUILD_TESTS "Build tests" ON).

I am not allowed to contribute to yoga due to the Meta CLA, thus I closed the PR where I propose such a change.

[NickGerleman]

What are build expectations of Conan? Yoga CMake structure is a bit messy, but if we don’t need bits which install Yoga headers to system, we could use https://github.com/facebook/yoga/blob/main/yoga/CMakeLists.txt directly.

[hedtke]

At the moment building and packaging is done via

cmake --build --target yogacore
cmake --install

this we install the lib and the headers