Add ability to control PulseAudio volume

I know, son, it might be hard to accept,
but you're imported.  Your true parents are wmstatus
and paswitch, from the desktop-tools family.

Also, fix unnecessary linking of optional dependencies.

config.h.in

@@ -6,6 +6,7 @@
 
 #cmakedefine HAVE_RESIZETERM
 #cmakedefine WITH_FFTW
+#cmakedefine WITH_PULSE
 
 #endif  // ! CONFIG_H