Passing -DBacktrace_LIBRARY='' doesn't work since the flag has been
renamed to Backtrace_LIBRARIES and isn't enough anyway to prevent the
llvm cmake files to *add a dep on all headers* in
${Backtrace_INCLUDE_DIRS}, ie /usr/local/include.
... which of course leads to builds blowing up whenever dpb(1) junking
kicks in.
Bulk builders: sorry it took me so long to get my head out of the sand.
Last report by naddy@
178b2ee6a0