Fix false libtool variable: _LIBS -> _LIBADD

_LIBS is not a libtool variable, thus the remotebackend module does not
get linked to libcurl. Fix this by using _LIBADD instead.
Use LUA_CFLAGS and LUA_LIBS instead of hardcoded paths.
Set automake variable HAVE_POLARSSL 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