clang has a -Weverything flag that, unlike "-Wall", will enable all compiler warnings. This includes possibly contradictory warnings. :) For new C code, -Weverything is a good place to start.

Unlike gcc, clang does not have good documentation for its warning options. The best sources I've found are the clang code itself (https://github.com/llvm-mirror/clang/blob/master/include/cla...) and http://fuckingclangwarnings.com/.