configure.ac: fix support for LOCKDIDM

Commit 7a8b7b4 introducing lockidm
support missed to use AC_SUBST() for a variable and provide it only
in prebuilt configure, so with the next autoreconf this variable
was lost and IDM support was no longer compiled.

Fixes: lvmteam/lvm2#98
Reported-by: Tom Prohofsky

configure.ac

@@ -1826,6 +1826,7 @@ AC_SUBST(BUILD_LVMLOCKD)
 AC_SUBST(BUILD_LOCKDSANLOCK)
 AC_SUBST(BUILD_LOCKDDLM)
 AC_SUBST(BUILD_LOCKDDLM_CONTROL)
+AC_SUBST(BUILD_LOCKDIDM)
 AC_SUBST(BUILD_DMFILEMAPD)
 AC_SUBST(CACHE)
 AC_SUBST(CFLAGS)