Rewrite Gnulib symbols to be in libcdecl's namespace.

The gnulib symbols are not properly prefixed for libcdecl internal
symbols (e.g. cdecl__blah).  This *will* cause problems when statically
linking against both libcdecl and another library that uses the same
gnulib modules.

Unfortunately, there's no simple way to determine what symbols need
prefixing without compiling all the gnulib objects.  The results can't
be distributed either, because they depend on the configuration.  So,
limited to simple tools and portable make rules, we hack together a
"new" config.h at build time that defines object-like macros to rewrite
the symbols, carefully ensuring that header dependencies are correct.