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But only when MKV is enabled.
reverts e75e49c0.

This a merge of the work done by Shaleen during GSOC 2017. See
https://code.videolan.org/GSoC2017/shalzz/vlc.gitSigned-off-by: Thomas Guillem <thomas@gllm.fr>

This a partial merge of the work done by Shaleen during GSOC 2017. See
https://code.videolan.org/GSoC2017/shalzz/vlc.gitSigned-off-by: Thomas Guillem <thomas@gllm.fr>

vlc arguments are passed by environment variable in order to don't mess
libfuzzer arguments handling:

- "VLC_TARGET" to pass the target name (or demux name).
- "V" to specify the verbose level.

This includes support for statically linked plugins. It vastly increases
the test iteration speed, which is critical for fuzz testing.
Furthermore, it is necessary for coverage-driven fuzz testing to work at
all.

This also provides a (manually compiled only) back-end for LLVM's
LibFuzzer using mostly the same code.

1) Debugging, regression testing or unguided fuzzing:
  - Make a normal build (debug and sanitization recommended).
  - Execute: "test/vlc-demux-run [demux name] <file path>"

2) American Fuzzy Lop run:
  - Make a *static* build with AFL as the toolchain.
  - (Where applicable) perform adequate religious luck granting
    offerings or other rites.
  - Run AFL with test/vlc-demux-run as the fuzzed executable.

3) LibFuzzer:
  - Make a preferrably static build with Clang as the toolchain.
  - Manually build test/vlc-demux-libfuzzer.
  - Run the executable with the LibFuzzer command line parameters syntax.