DRTVWR-418: Change Mac build_directory to build-darwin-x86_64

since we no longer support 32-bit Mac builds.

The old build-darwin-i386 directory name appeared in a shocking number of
files. Change CMake paths to use ${CMAKE_BINARY_DIR} -- or, when trying to
find the packages subdirectory, ${AUTOBUILD_INSTALL_DIR}. Change the rest to
at least look for build-darwin-*.