_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.
While there, also modify the install target. We should make sure to
install all openlibm*.h headers. There is still some work to be done:
openlibm_fenv_*.h still depends on some additional bits. I'd propose
that we eventually create an include/openlibm_cdefs.h that contains all
of the macros we need.
