configure: add LIBDRM to extralibs_avutil

When static linking programs using ffmpeg libraries, if linking against
libavutil, -ldrm is listed before -lavutil. This leads to linking failure
due to undefined reference of drmGetVersion() and drmFreeVersion().
This is why when pkg-config create libavutil.pc doesn't append -ldrm
after -lavutil.

Create LIBDRM=-ldrm in case libdrm is enabled and add $LIBDRM to
extralibs_avutil.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -5919,7 +5919,7 @@ enabled libcelt           && require libcelt celt/celt.h celt_decode -lcelt0 &&
                                die "ERROR: libcelt must be installed and version must be >= 0.11.0."; }
 enabled libcaca           && require_pkg_config libcaca caca caca.h caca_create_canvas
 enabled libdc1394         && require_pkg_config libdc1394 libdc1394-2 dc1394/dc1394.h dc1394_new
-enabled libdrm            && require_pkg_config libdrm libdrm xf86drm.h drmGetVersion
+enabled libdrm            && require_pkg_config libdrm libdrm xf86drm.h drmGetVersion && LIBDRM="-ldrm"
 enabled libfdk_aac        && { use_pkg_config libfdk_aac fdk-aac "fdk-aac/aacenc_lib.h" aacEncOpen ||
                                { require libfdk_aac fdk-aac/aacenc_lib.h aacEncOpen -lfdk-aac &&
                                  warn "using libfdk without pkg-config"; } }
@@ -7096,7 +7096,7 @@ source_path=${source_path}
 LIBPREF=${LIBPREF}
 LIBSUF=${LIBSUF}
 
-extralibs_avutil="$LIBRT $LIBM"
+extralibs_avutil="$LIBRT $LIBDRM $LIBM"
 extralibs_avcodec="$extralibs"
 extralibs_avformat="$extralibs"
 extralibs_avdevice="$extralibs"