configure: add libpulse to help output

Fixes Bug 72.

configure

@@ -178,6 +178,7 @@ External library support:
   --enable-libnut          enable NUT (de)muxing via libnut,
                            native (de)muxer exists [no]
   --enable-libopenjpeg     enable JPEG 2000 decoding via OpenJPEG [no]
+  --enable-libpulse        enable Pulseaudio input via libpulse [no]
   --enable-librtmp         enable RTMP[E] support via librtmp [no]
   --enable-libschroedinger enable Dirac support via libschroedinger [no]
   --enable-libspeex        enable Speex support via libspeex [no]