configure: fix libxavs check

libxavs may require pthreads and libm at link time, and without
said ldflags available as global extralibs, the check will fail.

Regression since 6dfcbd80.
Tested-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -6098,7 +6098,7 @@ enabled libx264           && { use_pkg_config libx264 x264 "stdint.h x264.h" x26
                                enable libx262; }
 enabled libx265           && require_pkg_config libx265 x265 x265.h x265_api_get &&
                              require_cpp_condition x265.h "X265_BUILD >= 68"
-enabled libxavs           && require libxavs "stdint.h xavs.h" xavs_encoder_encode -lxavs
+enabled libxavs           && require libxavs "stdint.h xavs.h" xavs_encoder_encode "-lxavs $pthreads_extralibs $libm_extralibs"
 enabled libxvid           && require libxvid xvid.h xvid_global -lxvidcore
 enabled libzimg           && require_pkg_config libzimg "zimg >= 2.3.0" zimg.h zimg_get_api_version
 enabled libzmq            && require_pkg_config libzmq libzmq zmq.h zmq_ctx_new