If you don't need/want the various Sanitizer runtimes (e.g. you don't
support sanitizers or already have versions provided with GCC) then
it's as easy as not downloading compiler-rt or removing it from the
projects/ directory before running CMake. The build should carry on
quite happily without it.
If you do need the sanitizers from compiler-rt, you can disable the
bits that are causing problems right now with "cmake
-DCOMPILER_RT_BUILD_BUILTINS=OFF<other options>". Of course, the
sanitizers themselves may suffer from similar issues -- you just won't
know until you get there.