✨ Add possibility to build static, and improved adding as a CMake subdirectory

Until now, adding shmdata as a submodule, or to be built along another project, needed to handle everything manually. See what is done in OSSIA Score for instance.

This MR improves this by mimicking what is done with the GLFW library. Multiple build flags, starting with SHMDATA_, have been renamed or added to allow for deactivating about everything except for the main library. And it is now possible to build shmdata as a static library using the SHMDATA_BUILD_STATIC flag.

See this example in Splash for a glimpse of how it can be used.