Merge branch 'jk/build-with-asan'

A recent update made it easier to use "-fsanitize=" option while
compiling but supported only one sanitize option.  Allow more than
one to be combined, joined with a comma, like "make SANITIZE=foo,bar".

* jk/build-with-asan:
  Makefile: allow combining UBSan with other sanitizers

diff --cc Makefile
Simple merge