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.
While there, make CMPLX() work with Clang by using compound literals.
Now that cimag*() uses __imag__, we can also just inline the unions.
There is no need for the separate types anymore.
Also just define CMPLX() unconditionally now, as we no longer pull in
the host's <complex.h>.
Ed Schouten