bump cocoa bindings
load VAO extension
add boost headers to cflags_cc for text_conversions test
add glx support
Merge pull request #564 from mapbox/load-extensions

Load extensions with *GetProcAddress
repair undef None GLX wrapper, pass 0 instead of None to GLX functions
Merge pull request #583 from mapbox/glx-undef-none

repair undef None GLX wrapper, pass 0 instead of None to GLX functions
check library versions against header symbols in static initialization function

fixes #580
cleanups
Merge branch 'master' into image-readers