SL-16040: Honor autobuild_{configure,build}_parameters variables.

The generic build.sh script honors these environment variables to specify
parameters to the autobuild configure and autobuild build steps, respectively.
Support them in the viewer-specific build.sh too.

In the generic build.sh, autobuild_configure_parameters allows specifying
command-line switches either for autobuild or, following --, for the
underlying tool (in our case, CMake). In order to support that variable the
same way here, we insert -- (as before) when autobuild_configure_parameters is
unset or empty, since the rest of the switches *we* specify are for CMake.

That means that, as with the generic build.sh, a non-empty
autobuild_configure_parameters override must precede any switches intended for
CMake with the -- separator.