configure: Restore doc option to disable building the documentation

Also add appropriate dependency declarations for perl and texi2html.

configure

@@ -1112,6 +1112,7 @@ CONFIG_LIST="
     $LIBRARY_LIST
     $PROGRAM_LIST
     dct
+    doc
     error_resilience
     fft
     gpl
@@ -1972,6 +1973,10 @@ avserver_deps="avformat fork !shared"
 avserver_select="ffm_muxer rtp_protocol rtsp_demuxer"
 avserver_extralibs='$ldl'
 
+# documentation
+perl_deps="doc"
+texi2html_deps="doc"
+
 # default parameters
 
 logfile="config.log"