Fix autotools cross-compiling support

Fix autotools cross-compiling support

This is a modified version of a patch sent to the sf.net
patch tracker:

<http://sourceforge.net/tracker/?func=detail&aid=2491190&group_id=48978&atid=454721>

After having disucssed this patch on IRC (#openvpn-discussions)
March 4, 2010, it was decided to accept this patch when not modifying
TARGET_* defines through out the code.  Further, in a mail comment
Alon Bar-Lev had some other comments of what would be needed to be done.

Mail reference:
<http://thread.gmane.org/gmane.network.openvpn.devel/3176>

This patch has been tested by bootstrapping the code on a RHEL4.6 box.
with the following autotools packages installed:
autoconf-2.59-5
automake-1.9.2-3
libtool-1.5.6-4.EL4.2

It builds cleanly and 'make check' passes.

Signed-off-by: David Sommerseth <dazo@users.sourceforge.net>
Acked-by: James Yonan <james@openvpn.net>
Acked-by: Alon Bar-Lev <alon.barlev@gmail.com>