Fixed issues running on TurboVNC with VirtualGL

These commits enable the use of GLFW applications running across TurboVNC+VirtualGL, whereas GLFW previously failed entirely.  Two issues are addressed, one of which was previously reported as https://github.com/glfw/glfw/issues/130

1. Xkb is made optional.  The Xkb extension is missing on a platform I have to support (RHEL6 running over TurboVNC and VirtualGL).   The deprecated function XKeycodeToKeysym is used as a fallback for when Xkb is not available.  Since this function is deprecated, to avoid compiler warnings, the call is only enabled at compile-time by disabling GLFW_USE_XKB. 
2. The call to ChooseFBConfigSGIX often fails, and now, rather than stopping with an error, an attempt is made to call glXGetFBConfigs as a fallback.