fix endianness detection with clang on osx 10.9

When compiling on OSX 10.9 using Apple's clang:

    In file included from numpy/core/src/npymath/npy_math.c.src:56:
    numpy/core/src/npymath/npy_math_private.h:78:3: error: typedef redefinition
    with different types ('union ieee_double_shape_type' vs 'union
    ieee_double_shape_type')

    } ieee_double_shape_type;
      ^

    numpy/core/src/npymath/npy_math_private.h:64:3: note: previous definition is
    here

    } ieee_double_shape_type;
      ^

    1 error generated.

    error: Command "clang -fno-strict-aliasing -fno-common -dynamic -arch i386
    -arch x86_64 -I/usr/local/include -I/usr/local/opt/sqlite/include -DNDEBUG
    -g -fwrapv -O3 -Wall -Wstrict-prototypes -Inumpy/core/include
    -Inumpy/core/include/numpy -Inumpy/core/src/private -Inumpy/core/src
    -Inumpy/core -Inumpy/core/src/npymath -Inumpy/core/src/multiarray
    -Inumpy/core/src/umath -Inumpy/core/src/npysort -Inumpy/core/include
    -I/usr/local/Cellar/python/2.7.7_1/Frameworks/Python.framework/Versions/2.7/include/python2.7
    -Inumpy/core/src/private -Inumpy/core/src/private -Inumpy/core/src/private
    -Inumpy/core/src/private
    -I/usr/local/Cellar/python/2.7.7_1/Frameworks/Python.framework/Versions/2.7/include/python2.7
    -c numpy/core/src/npymath/npy_math.c -o
    build/temp.macosx-10.9-intel-2.7/numpy/core/src/npymath/npy_math.o" failed
    with exit status 1

This is because `/usr/include/i386/endian.h` (included by `<endian.h>`) defines BYTE_ORDER, LITTLE_ENDIAN and BIG_ENDIAN without leading underscores. I'm not sure why this wasn't fixed earlier.
The patch is coming from https://dev.lsstcorp.org/trac/ticket/1926.