Add fake OSMAJORVERSION/OSMINORVERSION CPP flags

Add fake OSMAJORVERSION/OSMINORVERSION CPP flags

These really need to go away and are primarily used by older
unsupported platforms.

Here we fake these for Linux (4.15), freebsd (10.0), openbsd (6.2),
netbsd (8.0), and solaris (5.10).

I'm not sure about the Solaris one as I don't have any of them.
Basically, anything after Solaris 5.10, which is pretty ancient.

These should be considered temporary - all uses of these macros in the
code should be removed in favor of functionality-specific checks in
configure.