ffmpeg libmfx not found

Hi,

I'm trying to use hardware acceleration with ffmpeg on a Ubuntu server and when I'm trying to configure ffmpeg before compile, i got this error :

ERROR: libmfx not found

I'm running Ubuntu 18.10 and a i7 6700K CPU.

I follow those two procedures :

https://www.intel.com/content/www/us/en/cloud-computing/quicksync-video-ffmpeg-install-valid.html

https://software.intel.com/en-us/articles/build-and-debug-open-source-media-stack

Here are the key command and results that make me think that libmfx is ok :

$ pkg-config --list-all| grep mfx
libmfx                libmfx - Intel Media Server Studio SDK

$ pkg-config --modversion libmfx
16.5

$ pkg-config --libs libmfx
-L/opt/intel/mediasdk/lib/lin_x64 -lmfx -lva -lstdc++ -ldl -lva-drm -ldrm Cflags: -I/opt/intel/mediasdk/include -I/usr/include/libdrm

$ cat /usr/lib/pkgconfig/libmfx.pc 
prefix=/opt/intel/mediasdk
exec_prefix=${prefix}
libdir=${prefix}/lib/lin_x64
includedir=${prefix}/include
Name: libmfx
Description: Intel Media Server Studio SDK
Version: 16.5
Libs: -L${libdir} -lmfx -lva -lstdc++ -ldl -lva-drm -ldrm Cflags: -I${includedir} -I/usr/include/libdrm

I got the last ffmpeg sources through git and try to configure :

$ ./configure --enable-nonfree --enable-libmfx
ERROR: libmfx not found

I got this error in ffbuild/config.log :

check_pkg_config libmfx libmfx mfx/mfxvideo.h MFXInit
test_pkg_config libmfx libmfx mfx/mfxvideo.h MFXInit
pkg-config --exists --print-errors libmfx
check_func_headers mfx/mfxvideo.h MFXInit -L/opt/intel/mediasdk/lib/lin_x64 -lmfx -lva -lstdc++ -ldl -lva-drm -ldrm Cflags: -I/opt/intel/mediasdk/include -I/usr/include/libdrm
test_ld cc -L/opt/intel/mediasdk/lib/lin_x64 -lmfx -lva -lstdc++ -ldl -lva-drm -ldrm Cflags: -I/opt/intel/mediasdk/include -I/usr/include/libdrm
test_cc -L/opt/intel/mediasdk/lib/lin_x64 Cflags: -I/opt/intel/mediasdk/include -I/usr/include/libdrm
BEGIN /tmp/ffconf.gJsgAWPJ/test.c
    1   #include <mfx/mfxvideo.h>
    2   #include <stdint.h>
    3   long check_MFXInit(void) { return (long) MFXInit; }
    4   int main(void) { int ret = 0;
    5    ret |= ((intptr_t)check_MFXInit) & 0xFFFF;
    6   return ret; }
END /tmp/ffconf.gJsgAWPJ/test.c
gcc -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -DPIC -std=c11 -fomit-frame-pointer -fPIC -pthread -L/opt/intel/mediasdk/lib/lin_x64 Cflags: -I/opt/intel/mediasdk/include -I/usr/include/libdrm -c -o /tmp/ffconf.gJsgAWPJ/test.o /tmp/ffconf.gJsgAWPJ/test.c
gcc: error: Cflags:: No such file or directory
require libmfx mfx/mfxvideo.h MFXInit -llibmfx
check_lib libmfx mfx/mfxvideo.h MFXInit -llibmfx
check_func_headers mfx/mfxvideo.h MFXInit -llibmfx
test_ld cc -llibmfx
test_cc
BEGIN /tmp/ffconf.gJsgAWPJ/test.c
    1   #include <mfx/mfxvideo.h>
    2   #include <stdint.h>
    3   long check_MFXInit(void) { return (long) MFXInit; }
    4   int main(void) { int ret = 0;
    5    ret |= ((intptr_t)check_MFXInit) & 0xFFFF;
    6   return ret; }
END /tmp/ffconf.gJsgAWPJ/test.c
gcc -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -DPIC -std=c11 -fomit-frame-pointer -fPIC -pthread -c -o /tmp/ffconf.gJsgAWPJ/test.o /tmp/ffconf.gJsgAWPJ/test.c
gcc -Wl,--as-needed -Wl,-z,noexecstack -o /tmp/ffconf.gJsgAWPJ/test /tmp/ffconf.gJsgAWPJ/test.o -llibmfx
/usr/bin/ld: cannot find -llibmfx
collect2: error: ld returned 1 exit status
ERROR: libmfx not found

Well, I don't understand where could be the issue so any help and/or support will be warmly appreciated.

Cheers