configure: Allow linking to the gnutls library

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -162,6 +162,7 @@ External library support:
   --enable-avisynth        enable reading of AVISynth script files [no]
   --enable-bzlib           enable bzlib [autodetect]
   --enable-frei0r          enable frei0r video filtering
+  --enable-gnutls          enable gnutls [no]
   --enable-libopencore-amrnb enable AMR-NB de/encoding via libopencore-amrnb [no]
   --enable-libopencore-amrwb enable AMR-WB decoding via libopencore-amrwb [no]
   --enable-libopencv       enable video filtering via libopencv [no]
@@ -922,6 +923,7 @@ CONFIG_LIST="
     avserver
     fft
     frei0r
+    gnutls
     golomb
     gpl
     gray
@@ -2889,6 +2891,7 @@ check_mathfunc truncf
 # these are off by default, so fail if requested and not available
 enabled avisynth   && require2 vfw32 "windows.h vfw.h" AVIFileInit -lavifil32
 enabled frei0r     && { check_header frei0r.h || die "ERROR: frei0r.h header not found"; }
+enabled gnutls     && require_pkg_config gnutls gnutls/gnutls.h gnutls_global_init
 enabled libdirac   && require_pkg_config dirac                          \
     "libdirac_decoder/dirac_parser.h libdirac_encoder/dirac_encoder.h"  \
     "dirac_decoder_init dirac_encoder_init"
@@ -3180,6 +3183,7 @@ echo "libva enabled             ${vaapi-no}"
 echo "libvdpau enabled          ${vdpau-no}"
 echo "AVISynth enabled          ${avisynth-no}"
 echo "frei0r enabled            ${frei0r-no}"
+echo "gnutls enabled            ${gnutls-no}"
 echo "libcdio support           ${libcdio-no}"
 echo "libdc1394 support         ${libdc1394-no}"
 echo "libdirac enabled          ${libdirac-no}"