_LITTLE_ENDIAN and _BIG_ENDIAN are built-in on some platforms/versions.
Better use __ORDER_LITTLE_ENDIAN__, __ORDER_BIG_ENDIAN__ and __BYTE_ORDER__,
which are standard for gcc and clang, and define them when they are missing.
Also remove the special-case for FreeBSD, which is apprently not needed.
The current tradition of openlibm is to hide all of its internal symbols
into the reserved system namespace. CloudABI has a check in place to
ensure that its C library (which contains openlibm) to not place any
unwanted symbols into the public namespace. openlibm seems to leak
_scan_nan() in there, so we'd better add an additional underscore.
Milan Bouchet-Valat