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.
Instead of using all sorts of operating system specific constructs, we
can just query the compiler which byte order is being used. This has the
advantage that the code builds on new platforms without any tweaks.
