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>

configure

@@ -5791,7 +5791,14 @@ enabled libgsm            && { for gsm_hdr in "gsm.h" "gsm/gsm.h"; do
                                done || die "ERROR: libgsm not found"; }
 enabled libilbc           && require libilbc ilbc.h WebRtcIlbcfix_InitDecode -lilbc
 enabled libkvazaar        && require_pkg_config "kvazaar >= 0.8.1" kvazaar.h kvz_api_get
-enabled libmfx            && require_pkg_config libmfx "mfx/mfxvideo.h" MFXInit
+# While it may appear that require is being used as a pkg-config
+# fallback for libmfx, it is actually being used to detect a different
+# installation route altogether.  If libmfx is installed via the Intel
+# Media SDK or Intel Media Server Studio, these don't come with
+# pkg-config support.  Instead, users should make sure that the build
+# can find the libraries and headers through other means.
+enabled libmfx            && { use_pkg_config libmfx "mfx/mfxvideo.h" MFXInit ||
+                               { require libmfx "mfx/mfxvideo.h" MFXInit -llibmfx && warn "using libmfx without pkg-config"; } }
 enabled libmodplug        && require_pkg_config libmodplug libmodplug/modplug.h ModPlug_Load
 enabled libmp3lame        && require "libmp3lame >= 3.98.3" lame/lame.h lame_set_VBR_quality -lmp3lame
 enabled libnut            && require libnut libnut.h nut_demuxer_init -lnut