Fixed bug 2696 - Mac: fix display mode refresh rate calculation

Alex Szpakowski

SDL's Cocoa backend uses the CGDisplayMode API to get refresh rate information about a display mode, but CGDisplayModeGetRefreshRate will return 0 on most non-CRT monitors.

The only way I know of to get correct refresh rate information in OS X is via the CoreVideo DisplayLink API.
I have attached a patch which tries to use the CVDisplayLinkGetNominalOutputVideoRefreshPeriod function if CGDisplayModeGetRefreshRate fails, which fixes display mode refresh rate information on the monitors I tested.

The CVDisplayLink API requires linking with the CoreVideo framework, and the patch updates the various build files to do so.
Fixed Linux build