DRTVWR-418: Adjust for LL_VIEWER_CHANNEL coming in unquoted.

Evidently the LL_VIEWER_CHANNEL macro (defined on the compiler command line)
used to contain enclosing double quotes. Something changed (newer CMake
version?) so that the macro now expands as Second Life Release rather than as
"Second Life Release". That leads to syntax errors when it's used.

Add C++ preprocessor trickery to stringize the value of the macro.