FindFFMPEG: Support linking against static FFMPEG libraries on Linux

[nrusch]

Update FindFFMPEG module to facilitate linking against static FFMPEG libraries on Linux.

The changes do the following (currently only for static FFMPEG libraries):

• Add the -Wl,-Bsymbolic flag to each imported target's INTERFACE_LINK_OPTIONS.
• Using pkg-config to look up the transitive dependencies of each imported target, and including them in the target's INTERFACE_LINK_LIBRARIES, as well as populating INTERFACE_LINK_DIRECTORIES.

The second step could potentially be done for both static and dynamic FFMPEG libraries, but I kept the scope of this narrow to start with.

Fixes #60.

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: nrusch / name: Nathan Rusch (500e7cecdc43298b526f3a9d198cbcdc9fdb1929)
• :white_check_mark: login: tedwaine (e734d17cd59559ee46d017ec10d6d1ee7e558e59)

[tedwaine]

Thank you.