This is because all of these headers are included in direct style with
no subfolder so we need them to be in a quite public `include/` folder.
The Makefile installs in `include/openlibm/` but this only works with
extra `-I` flags that are quite annoying to enforce.
With this change, the only OpenLibm headers that are installed are named
`openlibm_*.h`, which is sufficient in terms of namespacing.
Also add a symlink libm.a -> libopenlibm.a.
Lephenixnoir