Restructure system PolarSSL detection

Based on the chapter "Automatic Dependencies with AC_ARG_WITH" of the
Autotools Mythbuster eBook, fix the system PolarSSL detection and
recognize both --with/without-system-polarssl properly. Furthermore,
set the HAVE_LIBPOLARSSL automake variable conditionally. Otherwise pdns will
use the included headers but link against the system polarssl resulting in
subtle breaks.
Link against bundled polarssl only if requested

This fixes a build error when using system PolarSSL