With .devel, use -W flags iff the compiler supports them.

With .devel, use -W flags iff the compiler supports them.

Don't base the question of whether to use a particular -W flag on
whether we're compiling with GCC or not, and on which version of GCC
we're using; some compilers, such as clang, also support many of those
-W flags.  Base it on whether the compiler complains if we use it (which
means, for clang, that we have to force it to exit with a non-zero exit
status if it doesn't like a flag; otherwise, we'll specify it, and clang
will warn about it on every compile).