fix-ltdl: Fix order-only hack with FreeBSD make.

fix-ltdl: Fix order-only hack with FreeBSD make.

FreeBSD make does not work well when undefined variables are used in
nested expansions.  When such an expansion appears in the prerequisites
of a target, FreeBSD make exits with a fatal error.  For example:

  A = $(var$(B))
  foo : $(A)

  % make
  Error expanding embedded variable.

This causes the order-only hack to fail because FreeBSD make does not
define .FEATURES.  The workaround is simple: add another variable which
is assigned the value of .FEATURES.  That variable can be used in the
nested expansion to avoid the error.