libdc1394-2 support for windows (MinGW)

With this patch, OpenCV can be build with firewire support (libdc1394-2) on the windows platform. At this time the libdc1394 library only supports the MinGW compiler and not any version of MSVC.

If libdc1394 has been installed as directed by its readme file, cmake should automatically detect all necessary libdc1394 header and library files in the MinGW installation location (/mingw /mingw32 /mingw64)

When not using a MinGW compiler on the windows platform, libdc1394 support will still be disabled and not available like before.

This patch is primary based on the following patch, supported by Philippe Foubert: http://code.opencv.org/issues/1967