clang level not detected on OS X Sierra for Xcode 9 in configure.ac changed "applellvm-5.1|applellvm-6.*|applellvm-7.*|applellvm-8.*)" to "applellvm-5.1|applellvm-6.*|applellvm-7.*|applellvm-8.*|applellvm-9.*)" and the build was successful
This has already been reported and fixed in the development Git version of valgrind. Whilst there is no current release that includes this fix, you can get hold of the development Git version here: http://valgrind.org/downloads/repository.html *** This bug has been marked as a duplicate of bug 383811 ***