v3.4/glfw: add blank import for glfw/include directory

The include directory contains files like xdg-shell-client-protocol.h
that are needed for the build. The blank import causes it to be included
when vendoring.

Fixes #422.

GitHub-Pull-Request: #423

v3.4/glfw: add initial implementation of GLFW 3.4 (#414)

See https://github.com/glfw/glfw/releases/tag/3.4.

Fixes #393.
Closes #400.

GitHub-Pull-Request: #414