log: allow color highlighting in Cygwin's mintty

Configure will detect the availability of the Windows' console functions and set
HAVE_SETCONSOLETEXTATTRIBUTE.  Meaning av_log will use those functions to
control colours.  When ffmpeg is run in Cygwin's mintty terminal emulator it
will not use colour highlighting in this case.

Mintty responds to the usual escape code colours (it even supports 256 colours).
Windows' cmd.exe does not.  Fortunately it seems that Cygwin's emulation layer
now translates the basic 16 colours into Windows' Console command functions.

That means that we can have av_log use the standard colour commands and let
ffmpeg print colours in both mintty and cmd.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>