Update how we configure gcc / clang command lines. (#103)

* 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.

bin/makefile-darwin.386-x

@@ -1,6 +1,6 @@
 # Options for MacOS, x86 processor, X windows
 
-CC = clang -m32 -std=gnu89 -funsigned-char -fno-strict-aliasing
+CC = clang -m32 $(CLANG_CFLAGS)
 OEXT=.o
 
 XFILES = $(OBJECTDIR)xmkicon.o \