* Move clang/gcc options into makefile-header.
This puts all of the shared options into a single place so that
we can easily update it.
* Add -fwrapv to the clang/gcc command lines.
This is a step along the way to fixing interlisp/medley#90.
Enabling `-fwrapv` will let overflow checks that rely upon undefined
behavior to work correctly. Without this, with modern versions of
clang and gcc, the overflow checks in the C code are eliminated
by the compiler, resulting in incorrect behavior on the part of
Maiko.
See the linked bug for details and discussion.
Hopefully, this usage of `-fwrapv` will go away once we're using
the correct compiler builtins to detect and handle overflow
correctly.
