configure: add --fatal-warnings option.

configure

@@ -78,6 +78,7 @@ Help options:
 Standard options:
   --logfile=FILE           log tests and output to FILE [config.log]
   --disable-logging        do not log configure debug information
+  --fatal-warnings         fail if any configure warning is generated
   --prefix=PREFIX          install in PREFIX [$prefix]
   --bindir=DIR             install binaries in DIR [PREFIX/bin]
   --datadir=DIR            install data files in DIR [PREFIX/share/ffmpeg]
@@ -2245,6 +2246,8 @@ for opt do
     ;;
     --help|-h) show_help
     ;;
+    --fatal-warnings) enable fatal_warnings
+    ;;
     *)
     optname="${opt%%=*}"
     optname="${optname#--}"
@@ -4377,7 +4380,10 @@ echo "#endif /* AVUTIL_AVCONFIG_H */" >> $TMPH
 
 cp_if_changed $TMPH libavutil/avconfig.h
 
-test -n "$WARNINGS" && printf "\n$WARNINGS"
+if test -n "$WARNINGS"; then
+    printf "\n$WARNINGS"
+    enabled fatal_warnings && exit 1
+fi
 
 # build pkg-config files