Warnings are errors on clang

We were setting -Wall -Werror when compiling with GCC to make sure that
we get rid of all compiler warnings as part of the standard build/test
process.  Now we do that for clang, too, for when we're developing on
Mac OS X.
Merge remote-tracking branch 'template/c-template' into develop

* template/c-template:
  Warnings are errors on clang