Improve cmake support, with installation

[Andre-LA]

Note: this was only tested on "x64 Native Tools Command Prompt" environment on Windows, using the clang compiler, next month Linux support will be tested.

With that, Nene it's not supposed to be used as a sub-directory of a game project (unless if the developer choice to), instead, Nene should be used just as SDL2 it's generally used: having a single (at least per-version) compiled version available on the system.

With basic support to CMake install (cmake --install), that's easier to accomplish, for instance, all the dlls required to run the game are easily available on the bin directory of the installation directory (that is, cmake's "prefix"), while the required headers for compilation are available on the include, and the nene.lib static library on lib directory.

Here's a example to compile a single C source file (using clang on Windows, consider that nene was installed using cmake --install build --prefix nene, that is, the final output it's on the nene directory):

> clang -fuse-ld=lld-link -Xlinker /subsystem:windows my_source.c -I nene\include\ -I nene\include\SDL2 -lSDL2main -lSDL2 -lSDL2_image -lSDL2_mixer -lSDL2_ttf -lnene -lshell32 -L nene\lib -o my_source.exe

This PR is related with #36.