DRTVWR-418: Use $LL_BUILD to set most switches in 00-Common.cmake.

Going forward, the intention is to set in 00-Common.cmake only switches not
already set for ALL viewer-related libraries in
https://bitbucket.org/lindenlab/viewer-build-variables/src/tip/variables.

To that end, remove all switches redundant with settings from that file.
Remove redundancies within 00-Common.cmake.
Remove cruft testing for gcc versions older than 4.3.