Allow choosing the underlying GL or GLES library via command-line flags

[hujiajie]

Currently the ANGLE backend cannot be built together with the OpenGL backend, while the expected behavior should be:

• Do not link with system GL or ANGLE at build time
• Dynamically load system GL when Aquarium is asked to use the OpenGL backend
• Dynamically load ANGLE when Aquarium is asked to use the ANGLE backend

When it comes to the WebGL port, there's a problem that Emscripten does not support dlopen("libGLESv2.so", flag) well. However, the WebGL port should always use "system GL", so the plan is to pass -lGLESv2 to the linker as an extra flag and make some C++ code guarded with #ifdef __EMSCRIPTEN__.

[JiangYizhou]

This is great! The branch build ANGLE with OpenGL on Windows by loading System library, but it's broken because of no rebase since last year. https://github.com/webatintel/aquarium/pull/6/files, see this file, src/aquarium-optimized/opengl/OpenGLFunctions.cpp.