configure: add missing pthreads extralib to the libilbc check

Debugged with the help of Reino Wijnsma.
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -5997,7 +5997,7 @@ enabled libgme            && { use_pkg_config libgme libgme gme/gme.h gme_new_em
 enabled libgsm            && { for gsm_hdr in "gsm.h" "gsm/gsm.h"; do
                                    check_lib libgsm "${gsm_hdr}" gsm_create -lgsm && break;
                                done || die "ERROR: libgsm not found"; }
-enabled libilbc           && require libilbc ilbc.h WebRtcIlbcfix_InitDecode -lilbc
+enabled libilbc           && require libilbc ilbc.h WebRtcIlbcfix_InitDecode -lilbc $pthreads_extralibs
 enabled libkvazaar        && require_pkg_config libkvazaar "kvazaar >= 0.8.1" kvazaar.h kvz_api_get
 # 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