compiler: don't override existing definitions.

It's common when integrating CCAN into existing projects that they define
such constants for themselves.  In an ideal world, the entire project
switches to one set of definitions, but for large projects (eg SAMBA)
that's not realistic and conflicts with the aim of making CCAN modules
easy to "drop in" to existing code.

(This is a generalization of Douglas Bagnell's patch sent in Message-ID:
<4DB8D00D.8000800@paradise.net.nz>).