* Move fetch & early return into fn build
* Pass ffmpeg_version as parameter
* Pass source_dir as parameter
* Create rustc_link_extralibs
* Use local variable instead of search()
* Pass OUT_DIR to the build function
* Return install_dir from build function
* Parametrize ffmpeg_major_version
* Remove duplicate import
* Rename `is_feature` to `optional`
* Replace Library::new with two separate functions
* Add AVHeader abstraction
* Add Library::enabled helper function
* Introduce helper fn cargo_feature_enabled
* Move switch and enable! to trait
This allows a simpler `configure.` invocation
* Move external libraries into static list
* Fix incorrect feature name for libdrm
The ffmpeg project changed their branch/tag naming structure. There is now a
branch for the major.minor version using the old naming scheme
(release/major.minor) and a tag called n{major}.{minor}[.{patch}].
This change broke the static builds which checked out no-longer-existing ffmpeg
branches with their old name.
