Don't generate/include config.h

See: https://bugzilla.mozilla.org/show_bug.cgi?id=868629

Our configuration header doesn't provide much information, so it may not be necessary to generate it at all, at this point at least.

This patch prevents the config header from being generated, and includes a "config.h" only if HAVE_CONFIG_H is defined -- This is not necessarily needed and can be removed if there are any objections to it.