Use WSTRINGIZE(), LL_TO_WSTRING(), wstringize() to produce required wide
strings. Use a lambda for callback that sends log file; use LLDir, if set, to
find the log file.

Introduce BUGSPLAT CMake variable to allow suppressing BugSplat.
Make BUGSPLAT CMake variable set LL_BUGSPLAT for C++ compilations.

Set viewer version macros on llappviewerwin32.cpp, llappviewerlinux.cpp and
llappdelegate-objc.mm -- because BugSplat needs the viewer version data, and
because the macOS BugSplat hook is engaged in an Objective-C++ function we
override in the app delegate.