modules/video_output/opengl.c: Allow opengl provider override at runtime.(Original patch by Enrique Osuna <enrique.osuna a gmail d com>).

Reverse previous workaround for pthreads lib having GC2 suffix - that
is actually normal; patch should instead go into x264 to make it ignore
the presence of a local pthread lib under cygwin when doing configure tests.

Contribs: take libebml and libmatroska from released versions... Way easier to not compile it everytime.

Ignore directories in --sub-autodetect-path that match the directory
of the current file (But it does not works with symbolic links).

Fixed an overflow when more than MAX_SUBTITLE_FILES are detected.
Fixed a bunch of memory leaks.
Fixed removing sub/idx pair. (sizeof() != strlen on char*)
Clean up/Simplify (asprintf/calloc are usefull)
Cosmetics

selected one with --sub-file

 - as a standalone file
 - as a kind of subtitle (even if it is a video track).
CD-G disk are not supported.

Please, report any file incorrectly decoded. (I have 0 sample with
circular scrolling)

In subtitle_Detect, do not add as a subtitle the file for which we
are doing the detection.
 (subtitle_Detect("file.sub") returned "file.sub")

Re-enables media library.
Fixes #1047 (nth attempt).

This means ICMP errors will be ignored, which is a Bad Idea.
But people are likely to complain about how it breaks their setup if we
don't ignore ICMP anyway.