This fixes warnings from clang-tidy like this:
parameter 'v_max' is const-qualified in the function declaration;
const-qualification of parameters only has an effect in function definitions
Since values (rather than references or pointers) don't need to be
const, they don't need to be marked that way in the function declaration.
Clang 4.0 introduced -Wdouble-promotion, so check for and disable
that on clang as we do on gcc.
The old style casts warning is already disabled globally in this file
for clang, so it doesn't need to be re-disabled within this scope.
