Correct endianness detection for OS X universal builds.

The configure script correctly detects universal compilation (e.g.,
`clang -arch x86_64 -arch i386`) and defines neither WORDS_BIGENDIAN nor
WORDS_LITTLEENDIAN. However, the build then errors out because the
endianness checks assume that at least one of WORDS_BIGENDIAN and
WORDS_LITTLEENDIAN are defined. Using the standard OS X endianness
macros fixes this.