config.mak: split up CFLAGS line

I'm going to be adding more to this in a minute.
config.mak: turn off zero-length-format warnings

The gcc warning for this is bogus, as it assumes format
functions have no useful side effects (e.g., printing a
newline). We've decided upstream that people should just
turn off the warning.
config.mak: add a COMPILER option

This makes it a little easier to build with clang.
config.mak: silence some clang warnings

Without -Qunused-arguments, clang complains that we gave it
redundant "-I" arguments.

Without -Wno-parentheses-equality, clang complains of:

  if ((x))

which is indeed poor form, except that in some cases the
code does not look like that at all, but is generated via
macro expansion. And the macro is correct to surround itself
in () for precedence safety.
merged: widen topic size

Some topic names can be quite long, and since I use a pretty
wide terminal these days, allocating 30 characters shouldn't
hurt.