It is included in Wall so we'll need to either fix them or suppress the warning when building those libraries.
src/third_party/zlib-1.2.8/inflate.c:1507:61: error: shifting a negative signed value is undefined [-Werror,-Wshift-negative-value]
if (strm == Z_NULL || strm->state == Z_NULL) return -1L << 16;
1 error generated
WiredTiger triggers clang 3.7's Wshift-negative-value warning