configure: enable PIC on s390(x)

The s390 architecture requires shared libraries to be built in PIC mode.
Otherwise applications will get wrong relocations at run-time, leading
to confusing segmentation faults.

CC: libav-stable@libav.org
(cherry picked from commit 5ddc9f5052316608799b932c604f9e7561f8ce24)
Revert "Add libx265 encoder"

cf. the discussion following
https://lists.libav.org/pipermail/libav-devel/2014-March/056894.html

This reverts commit 50ea93158d4c480f64069e8bd1da388486dcf4ba.

Conflicts:
	doc/general.texi
	libavcodec/version.h
Update default FATE URL for release/10