Newer versions of GLPK no longer support the original lpx_ API.
Since the use of GLPK is optional, there is little point in supporting
two APIs.  Switch to the new API and check that it is available.

Signed-off-by: Thomas Kahle <thomas.kahle@ovgu.de>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>