macro: add CONST_MAX() macro

macro: add CONST_MAX() macro

The CONST_MAX() macro is similar to MAX(), but verifies that both
arguments have the same type and are constant expressions. Furthermore,
the result of CONST_MAX() is again a constant-expression.

CONST_MAX() avoids any statement-expressions and other non-trivial
expression-types. This avoids rather arbitrary restrictions in both GCC
and LLVM, which both either fail with statement-expressions inside
type-declarations or statement-expressions inside static-const
initializations.

If anybody knows how to circumvent this, please feel free to unify
CONST_MAX() and MAX().