If you fail to specify a --llvm-symbolizer value when building with --sanitize=thread, you'll get an UnboundLocalError:
scons VARIANT_DIR=thread CC=`which clang-3.6` CXX=`which clang++-3.6` --sanitize=thread -j7 --allocator=system --opt=off core
...
UnboundLocalError: local variable 'tsan_options' referenced before assignment:
File "/home/s/code/mongo/mongo/SConstruct", line 2202:
env = doConfigure( env )
File "/home/s/code/mongo/mongo/SConstruct", line 1828:
tsan_options += "suppressions=\"%s\" " % myenv.File("#etc/tsan.suppressions").abspath