libpam-ssh fails to cross build from source, because configure.ac uses
AC_RUN_IFELSE without a cross compilation default in several occasions.
These checks really cannot be run during cross compilation, so I'm
proposing alternatives.

The first one is used to find the library. I propose degrading to
AC_LINK_IFELSE for cross builds only. Native builds shall continue to
use AC_RUN_IFELSE.

The second and third one verify that RSA works and that the openssl
header version matches the shared library version respectively. These
are sanity checks and I propose that we assume it success in cross
compilation scenarios.

With these changes, libpam-ssh can be cross built and I'm attaching them
as a patch for your convenience.

Helmut