Support for automatic graphics switching on OS X

Enables the context to participate in automatic graphics card switching.
This means that GLFW won't force switching to the discrete graphics 
card on any Mac that has dual graphics cards available.  
This will result in lower power consumption for GUI-style apps where
the integrated graphics are more than adequate, but may be ignored
for heavier applications such as games, where the user most likely
wants to take advantage of the more powerful discrete graphics chip.  

For this functionality to actually be active, the Apps' Info.plist
file still needs to be edited to contain: 
<key>NSSupportsAutomaticGraphicsSwitching</key><true/>
otherwise it continues with default behaviour of forced discrete gfx.

Note that switching graphics chipsets also incurs a ~200ms penalty on 
app startup, which is avoided by having this attribute set.
Further info: 
https://developer.apple.com/library/mac/technotes/tn2229/_index.html