configure: use pkg-config to detect libbluray

The current configure fails when static libbluray is compiled with libxml2
support.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit baa650cc7946a9eb1cf5a083f61a581a97122f03)
Signed-off-by: Timothy Gu <timothygu99@gmail.com>

Conflicts:
	configure
avidec: Let the inner dv demuxer take care of discarding

Fixes ticket #1796.
(cherry picked from commit c8f0b20b4a6bb6691928789d83e4b02896969848)

Conflicts:
	libavformat/avidec.c
Merge remote-tracking branch 'TimothyGu/release/1.2' into release/1.2

* TimothyGu/release/1.2:
  configure: use pkg-config to detect libbluray

Merged-by: Michael Niedermayer <michaelni@gmx.at>