In particular, OpenBSD already defines __GNUC_PREREQ__, __BEGIN_DECLS,
and __warn_references. Hiding the definitions behind #ifndef guards
avoids warnings when building.
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.
