configure: remove libzvbi GPL dependency

The COPYING.LIB file in the zvbi source tree as well as libzvbi.h references
the GNU Library General Public License version 2 since version 0.2.28.
Signed-off-by: Marton Balint <cus@passwd.hu>

configure

@@ -4886,7 +4886,6 @@ die_license_disabled gpl libx264
 die_license_disabled gpl libx265
 die_license_disabled gpl libxavs
 die_license_disabled gpl libxvid
-die_license_disabled gpl libzvbi
 die_license_disabled gpl x11grab
 
 die_license_disabled nonfree libaacplus
@@ -5573,7 +5572,9 @@ enabled libxavs           && require libxavs xavs.h xavs_encoder_encode -lxavs
 enabled libxvid           && require libxvid xvid.h xvid_global -lxvidcore
 enabled libzimg           && require_pkg_config zimg zimg.h zimg_get_api_version
 enabled libzmq            && require_pkg_config libzmq zmq.h zmq_ctx_new
-enabled libzvbi           && require libzvbi libzvbi.h vbi_decoder_new -lzvbi
+enabled libzvbi           && require libzvbi libzvbi.h vbi_decoder_new -lzvbi &&
+                             { check_cpp_condition libzvbi.h "VBI_VERSION_MAJOR > 0 || VBI_VERSION_MINOR > 2 || VBI_VERSION_MINOR == 2 && VBI_VERSION_MICRO >= 28" ||
+                               enabled gpl || die "ERROR: libzvbi requires version 0.2.28 or --enable-gpl."; }
 enabled mmal              && { check_lib interface/mmal/mmal.h mmal_port_connect -lmmal_core -lmmal_util -lmmal_vc_client -lbcm_host ||
                                 { ! enabled cross_compile && {
                                     add_cflags -isystem/opt/vc/include/ -isystem/opt/vc/include/interface/vmcs_host/linux -isystem/opt/vc/include/interface/vcos/pthreads -fgnu89-inline ;

doc/general.texi

@@ -173,12 +173,6 @@ Go to @url{http://sourceforge.net/projects/zapping/} and follow the instructions
 installing the library. Then pass @code{--enable-libzvbi} to configure to
 enable it.
 
-@float NOTE
-libzvbi is licensed under the GNU General Public License Version 2 or later
-(see @url{http://www.gnu.org/licenses/old-licenses/gpl-2.0.html} for details),
-you must upgrade FFmpeg's license to GPL in order to use it.
-@end float
-
 @section AviSynth
 
 FFmpeg can read AviSynth scripts as input. To enable support, pass