SL-821, SL-826: Use BUGSPLAT_DB from environment on Windows and Mac.

On TeamCity, set BUGSPLAT_DB from build-secrets.

Use the presence of $BUGSPLAT_DB, rather than a new CMake BUGSPLAT option, to
control whether CMake searches for BugSplat -- and passes LL_BUGSPLAT into C++.

When BUGSPLAT_DB is present, make viewer_manifest.py set "BugSplat DB" in
build_data.json, and "BugsplatServerURL" in Mac Info.plist.

Make llappviewerwin32.cpp read "BugSplat DB" from build_data.json.

Add placeholders for Mac hooks to suppress BugSplat prompt and send
SecondLife.log.