configure: Added require alternative for libmfx to support alternate installation options

Purpose: Added require alternative for libmfx in the case that pkg-config
cannot find libmfx.  On Linux, most people likely get libmfx via
https://github.com/lu-zero/mfx_dispatch , but on Windows, the most
well-known way to get libmfx is via the Intel Media SDK, which
provides a static build of libmfx.lib and also provides the source
code for building libmfx yourself.  If built this way, there are no
pkg-config files to be found.

Comments:

-- configure: Altered enabled libmfx step to use use_pkg_config()
   instead of require_pkg_config(), and, if use_pkg_config() fails, it
   falls back to require().  Also added explanatory comment.  Note
   that the reason that require() is passed -llibmfx as the last
   argument, instead of -lmfx, is the file name for the library
   produced from the Intel Media SDK starts with "libmfx".
   Apparently, the filename for the library produced via
   https://github.com/lu-zero/mfx_dispatch

 starts with "mfx".

Signed-off-by: Aaron Levinson <alevinsn@aracnet.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>