Add support for offline builds

When the LWJGL_BUILD_OFFLINE environment variable is set to true/on/yes, binary dependencies will not be downloaded. This is useful when working offline or when custom binaries are being used.
Update README
Always clean target folder when generating javadoc
Drop LWJGL 2 compatibility API
Remove platform from Sys

It did nothing and was only implemented for Windows. Will be reconsidered if there's ever a good reason to have a platform abstraction.
Update libffi headers
Add Javadoc support for @since
Update glfw3native.h to latest version
Output constant blocks with one element in a single line
Detect \# as an escape for #

Useful for 3rd-party links inside JavaDoc.
Update glfw3.h to latest version