Log Message:
chat/libotr, chat/irssi: Pass through libgcrypt-config on cross.

This is kludgey, on two levels:

1. We are relying on the _cross-built_ libgcrypt-config to be a shell
   script that can be executed at build-time.  (Why can't we just use
   pkg-config?  Downstream packages' configure scripts have to be
   patched to be taught to do this -- the libgcrypt.m4 autoconf library
   that gcrypt ships with doesn't know how.)

2. Knowing where the libgcrypt-config lives should be defined in one
   place, security/gcrypt/buildlink3.mk, not in every user like
   textproc/libxml2, chat/libotr, chat/irssi, &c.

But this will get irssi cross-building, at least.

No change to native build because conditional on USE_CROSS_COMPILE.