Use CMake for big-endian test

[ekroon]

The test that was in the source code itself was not working on OSX and incorrectly used big-endian. As CMake is already used, I changed the code to use that.

[dgrigsby]

Here's the definition code:
#define IS_BIG_ENDIAN (*(uint16_t *)"\0\xff" < 0x100)

And here's the code in endoders.c (similar in loaders.c):
#ifdef IS_BIG_ENDIAN
...
#else
...
#endif

Is the issue simply that we needed to use #if not #ifdef? (!)

[PJK]

Thank you, @ekroon, I find this approach much better.

I will revise the usage in #10 , thanks for pointing that out, @dgrigsby