Always include <config.h> rather than "config.h".

Always include <config.h> rather than "config.h".

This can prevent bizarre failures if, for example, you've done a
configuration in the top-level source directory, leaving behind one
config.h file, and then do an out-of-tree build in another directory,
with different configuration options.  This way, we always pick up the
same config.h, in the build directory.