DRTVWR-447: Introduce explicit CMake BUGSPLAT_DB variable.

Define the CMake cache variable, with empty string as its default.

Make build.sh pass the BUGSPLAT_DB environment variable as a CMake
command-line variable assignment.

Change CMake 'if (DEFINED ENV{BUGSPLAT_DB})' to plain 'if (BUGSPLAT_DB)'.

Make CMake pass new --bugsplat switch to every one of SIX different
invocations of viewer_manifest.py.

Give llmanifest.main() function an argument to allow supplementing the base
set of command-line switches with additional application-specific switches.

In viewer_manifest.py, define new --bugsplat command-line switch and pass to
llmanifest.main(). Instead of consulting os.environ['BUGSPLAT_DB'], consult
self.args['bugsplat'].