Do not export internal symbols from a unix shared library.
Merge pull request #54 from johnbartholomew/unix-so-hide-internal-symbols

Do not export internal symbols from a Unix shared library.
Removed centered flag.
Removed APPLE as UNIX is true on OS X.
Added setting of _NET_WM_PID.
Merged GLX error code into library struct.
TLS key cleanup.
Cleanup.
Fixed sharing between windowed and fullscreen windows.
Explicitly choose closest policy.
Replaced expensive IOKit function.
Initial OS X fullscreen iconification work.

glfwIconifyWindow now pulls the view out of fullscreen mode and restores
the original display mode, reversing the process when deminiaturized.
Moved OpenGL framework loading to NSGL module.
Formatting.