This makes it possible to:
- finally use Magnum as a CMake subproject on Windows and have your
executables not fail to run with a "DLL missing" error (and the
setting is put to cache so superprojects just implicitly make use of
that)
- run tests on Windows without having to install first
- use dynamic plugins from a CMake subproject on any platform without
having to install first or load them by filename --- and the plugin
directory is now easily discovered as relative to
libraryLocation() of the library implementing given plugin interface
Three new builds (GL, GLES2, GLES3), Vulkan is now built with just MSVC
2019; removed 2015 RT builds because nobody cares and axed also GLES
2015 builds. In total it's one build less (well, the RT ones are
really slow, so way more than that in fact). Yay!