DRTVWR-418: Another attempt to generate Mac debug symbols.

The viewer's 00-COMPILE-LINK-RUN.txt recommends passing -gdwarf-2 to the Mac
compiler, and so we've been doing ever since before the viewer-build-variables
repo was engaged. Now we discover that when CMake sees -gdwarf-2, it removes
the -g switch entirely. It also removes it when you pass plain -g. Only when
you pass -gdwarf-with-dsym or just -gdwarf does CMake pass plain -g to the
compiler. Change -gdwarf-2, if specified, to -gdwarf so we at least get -g.