Restores [f1b4d1d17200] functionality with a slight change to keep
'make dist' working (see backout in [6b38124a5b81]).

Automake is too much voodoo for me at this time, so I let it keep
VERSION. mutt.h defined MUTT_VERSION as VERSION and the code used that,
so I removed MUTT_VERSION from mutt.h and put it into config.h via
configure.ac.  A couple of tweaks were needed elsewhere.  This restores
the fancy-versioning feature within mutt and keeps 'make dist' happy.