Improve CMakeLists.txt to hook pkg-config

Add json-c dependency
Implement json-based assets loading

Add pack objects (ammo, health, armor).
Add sample assets
Backport font loading

Remove fontconfig dependency
Change c++11 standard to gnu++11 to build with MinGW
Backport GLEW library dependency

GLEW makes it possible to use OpenGL3 on Windows platform
without explicit opengl functions loading.