add source patch for freeimage

this makes minor adaptions to two  FreeImage source files that allow it to pass through the less permissive clang++ compiler

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
add clang-libc++ formula for FreeImage

Automates everything there is to do to build FreeImage for clang-libc++

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>