configure: add 'warn' function

The 'warn' function records a warning message for display after other
informational messages.

Originally committed as revision 23760 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -265,6 +265,11 @@ echolog(){
     echo "$@"
 }
 
+warn(){
+    log "WARNING: $*"
+    WARNINGS="${WARNINGS}WARNING: $*\n"
+}
+
 die(){
     echolog "$@"
     cat <<EOF
@@ -3069,6 +3074,8 @@ echo "#endif /* AVUTIL_AVCONFIG_H */" >> $TMPH
 
 cp_if_changed $TMPH libavutil/avconfig.h
 
+test -n "$WARNINGS" && printf "\n$WARNINGS"
+
 # build pkg-config files
 
 pkgconfig_generate(){