Add headers and pkg-config package files
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This PR aims to expand the utility of these images (see more detailed rationale below) by adding necessary development files to facilitate compiling downstream software against provided FFmpeg libraries.
The following are added:
- `libav*`, `libpostproc`, `libsw*` headers to `/usr/local/include`;
- `libav*`, `libpostproc`, `libsw*` package files to `/usr/local/lib/pkgconfig` (or `lib64` where appropriate);
- Unversioned symlinks to versioned shared objects to `/usr/local/lib/` (or `lib64` where appropriate), e.g. `/usr/local/lib/libavcodec.so -> libavcodec.so.58`; without these linker flags generated by `pkg-config --libs` won't actually work.
The `.scratch*` templates are excluded from these changes since it's hard to
imagine compiling downstream software against the barebones `-scratch` images.
### Rationale
I maintain a [Rust binding for FFmpeg](https://github.com/zmwangx/rust-ffmpeg) as well as software depending on that. I need to test the binding against multiple versions of FFmpeg, but building them, and more importantly, keeping up with new releases is a pretty significant undertaking. This repo is a godsend, but it's not currently usable since headers and pkg-config package files are excluded from the final stage. Any other downstream software that compiles against FFmpeg would face the same problems as I do. It appears there are past issues over this, too: #88, #215 (open). Since there's already a lot of effort poured into this repo, it would be awesome if downstream software that not only use the ff* executables could also benefit.
I understand that this repo aims to provide minimalist images, but this change adds ~1.3MB to 45MB-1.5GB images, which isn't meaningful bloat, and expands the utility quite considerably.
In case you're uncomfortable with adding these, would you be open to maybe adding a single `-devel` template with these? Thanks.