Rudimentary support for CMake

[unmanned-player]

This patch adds CMake build support for various target environments and compilers. The CMake builds and compiles all demo code against SDL-1.2, SDL-2.0 X11, Wincon and DOS. It works with GNU, OpenWatcom and MSVC compilers.

FindSDL2.cmake was added as I don't know how to use find_package() in CONFIG mode. While I've heard of people using PDCurses on Apple computers, I do not own one to verify if it works.

[Mizux]

You may want to use: or CMAKE_C_COMPILER_ID MATCHES clang) to also catch Apple/ARM/Cray/XL/IBM clang based compiler.s..

ref: https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_COMPILER_ID.html https://cmake.org/cmake/help/latest/command/if.html#matches