[trac1687] make man page generation on by default

and change help to say how to disable it.

Not sure if want to do this or not. The plan in for
tarballs to include generated man pages anyways, so maybe this default
is not needed.

configure.ac

@@ -906,8 +906,8 @@ AC_SUBST(PERL)
 AC_PATH_PROGS(AWK, gawk awk)
 AC_SUBST(AWK)
 
-AC_ARG_ENABLE(man, [AC_HELP_STRING([--enable-man],
-  [regenerate man pages [default=no]])], enable_man=$enableval, enable_man=no)
+AC_ARG_ENABLE(man, [AC_HELP_STRING([--disable-man],
+  [don't generate documentation])], enable_man=$enableval, enable_man=yes)
 
 AM_CONDITIONAL(ENABLE_MAN, test x$enable_man != xno)