[master] make --disable-man work

This was broken due to misuse of setting.
(--disable-man caused it to be enabled still.)

This may be changed in trac1687 branch to have different default behavior.

configure.ac

@@ -907,7 +907,7 @@ 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=yes, enable_man=no)
+  [regenerate man pages [default=no]])], enable_man=$enableval, enable_man=no)
 
 AM_CONDITIONAL(ENABLE_MAN, test x$enable_man != xno)