Minor changes to support Mac OSX (tested on 10.10)

[jensp]

These commits should fix untrunc for OSX. I don't really know how to program and I found the header file on a github gist, but it's in the public domain and the content seems simple enough. Maybe this will help some poor souls stuck on Apple's OS.

[spetzreborn]

Verified to still build on linux.

[RSully]

This may be helpful: https://gist.github.com/panzi/6856583

[mkoderer]

Fix works fine for me. Can we merge it?

[ezhes]

These changes allowed me to build on OSX Sierra! I'd approve them. I did have to make this change in file.cpp to get it to build

[slhck]

Actually, for OS X, you could make it easier by installing ffmpeg through Homebrew. Then, all that's needed is a -L/usr/local/lib flag for g++.

If you get the error error: use of undeclared identifier 'avcodec_alloc_frame', you have to replace all occurrences of avcodec_alloc_frame with av_frame_alloc in the source.

EDIT: This is the case for the libav libraries of FFmpeg 3.0, which is bundled with Homebrew.

[alejandroiglesias]

@slhck your instructions worked very well. Thanks!

[Letme]

Can we merge that in?