Use modern CMake best practices & VL superbuild

[MicBosi]

Overview:

• VL should be easy to include into external CMake superbuilds
• VL should use standard CMake to build it's external dependencies

Overhaul project structure to build subprojects and plugin dependencies using modern CMake and make VL, ie all these and GDCM. PhysX and CEGUI are probably best kept truly external.

Apparent CMake native support as of Feb 2020

• [Y] FreeType 2.10
• [Y] libpng
• [Y] zlib
• [Y] GDCM
• [Y] libtiff 4.1
• [N] libtiff 3.9
• [N] jpeg-9d

Collection of CMake-fied libraries we could depend on for a VL superbuild:

• https://github.com/LuaDist/ (not sure how up to date they are though)
• https://subscription.packtpub.com/book/application_development/9781788470711/8/ch08lvl1sec85/using-the-superbuild-pattern

References:

• http://www.saoe.net/blog/using-cmake-with-external-projects/
• https://pabloariasal.github.io/2018/02/19/its-time-to-do-cmake-right/
• https://cliutils.gitlab.io/modern-cmake/
• https://gitlab.kitware.com/cmake/community/-/wikis/doc/tutorials/How-to-create-a-ProjectConfig.cmake-file
• https://coderwall.com/p/y3zzbq/use-cmake-enabled-libraries-in-your-cmake-project
• https://stackoverflow.com/questions/16398937/cmake-and-finding-other-projects-and-their-dependencies/16404000
• https://www.youtube.com/watch?v=bsXLMQ6WgIk
• https://www.youtube.com/watch?v=rLopVhns4Zs

Ideally fetch external libs at configure time not at build time:

• https://cliutils.gitlab.io/modern-cmake/chapters/projects/fetch.html
• https://cmake.org/cmake/help/latest/command/find_package.html#full-signature-and-config-mode
• https://blog.kitware.com/cmake-superbuilds-git-submodules/
• https://cmake.org/cmake/help/v3.10/module/ExternalProject.html

Issues:
Currently zlib and others are compiled and included using a custom build not their own native CMake config.