Propagate cmake arguments to superbuild dependencies

[JonasVautherin]

This propagates the cmake arguments to the superbuild dependencies. It works by extracting the arguments into ${CL_ARGS} (before project() is called!), and then propagating that to the dependencies (through build_target.cmake).

The CMAKE_ARGS defined in each dependency's CMakeLists.txt are overriding values. For instance, we always want to build our dependencies as static libs, and therefore we set BUILD_SHARED_LIBS=OFF, no matter what the command line argument was. That's why ${CL_ARGS} appears at the top of the lists of arguments.

Note that we still need to manually set CMAKE_PREFIX_PATH and CMAKE_INSTALL_PREFIX, because they come from our DEPS_INSTALL_PATH option, and are therefore independent from the cmake command line arguments.

As an example, if we now set an Android-specific argument like ANDROID_STL, it automatically gets propagated to all our dependencies.

I believe that's an improvement, but I'm open to discussion if you think it is convoluted, @julianoes 😅.

[julianoes]

I suppose that's fine.

[JonasVautherin]

Finally passed the CI, there were connection issues yesterday.

Two things to note:

• I had to hardcode tinyxml2 to CMAKE_BUILD_TYPE=Release, because otherwise it generates libtinyxml2d.a and Ubuntu 16 doesn't find it. We could probably fix the find module, but I think it's not a big deal anyway.
• The path given to CMAKE_TOOLCHAIN_FILE now has to be absolute, because it is propagated to the dependencies. For some reason it was working (and is still working in current develop) with a relative path, but I really don't understand why it works. Or why the relative path doesn't work anymore.

@julianoes: As mentioned already, I'm not completely sure about this change. I like the idea of not having to go through all the dependencies everytime I want to support a new option (like -DANDROID_STL in this case), but in the other hand this solution is not as clean as I was hoping it to be. Your call then.

[julianoes]

Let's try it!