Remove {cdefs,types}-compat.h from openlibm.h

[staticfloat]

This is an alternate, perhaps better way to close #41. Also reverts the previous "fix".

[staticfloat]

Note that I'm being a little cavalier with my definition of __pure2. I figured that if not having __const__ is good enough for Linux and Windows, it's good enough for BSD as well. Although most of the checks for compatible gcc versions are checking 2.x releases, so perhaps we can just clean those checks out as well and everyone can get in on the __const__ goodness nowadays.

[JeffBezanson]

This looks like a good idea.