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.
<sys/cdefs.h> is not a standard header. Instead, we'd better pull in a
common header like <stdio.h>. It is very likely that such a header
already provides the necessary bits.
skip END and add .drectve export in assembly versions
uncomment __strong_reference definition from cdefs-compat.h
use weak references in place of strong references on Mac
add DLLEXPORT to all strong references
Fixes Julia issue #6777
