Don't report error if WORDS_BIGENDIAN is undefined -- config.h might
just /* #undef */ it.  Just assume undefined -> false.

Add default rules for .cc (and .C and .cpp) files -- not all versions of make
have them built-in.

We're little-endian if WORDS_BIGENDIAN is defined (in config.h) and set to 0,
so use #if rather than #ifdef to test endian-ness.