Improve compile-time detection of host endianness.
Improve compile-time detection of host endianness.

Should fix issue #86.

shihchinw/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3