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.

(Original bug report: http://trac.macports.org/ticket/40401)