[libcxx] Limit catopen usage to unix-like OSes

[bcraig]

Summary

Operating systems that are not unix-like are unlikely to have access to catopen. Instead of black-listing each one, we now filter out all non-unix operating systems first. We then exclude the unix-like operating systems that don't have catopen. _WIN32 counts as a unix-like operating system because of cygwin.

[ed]

I'm usually not a big fan of using definitions such as __unix__, because it is pretty vague and defined inconsistently. That said, if you're going to use this definition, then there is some good news: CloudABI doesn't define it, meaning that you could just remove it from this list.

[bcraig]

CloudABI doesn't define unix, so stop blacklisting it explicitly and let the whitelist filter it out.

[mclow.lists]

I don't have a problem with this change (assuming that it is correct, and it looks correct), but I think it should stay in __config, with all the other configuration tests. (There are other configuration tests in the .cpp files, but users never see those).

FWIW, looking at locale, I think that code can be significantly cleaned up.

[bcraig]

Moved the CATOPEN config check back to __config. Saving a larger locale refactor for later (but oh boy is it coming).

[mclow.lists]

This looks good to me.

Saving a larger locale refactor for later (but oh boy is it coming).

Looking forward to it.

include/__config

 #define _LIBCPP_LOCALE__L_EXTENSIONS 1
 #endif
 
-#if !defined(_WIN32) && !defined(__ANDROID__) && !defined(_NEWLIB_VERSION) && \
-    !defined(__CloudABI__)
+#if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
+// Most unix variants have catopen.  These are the specific ones that don't.
+#if !defined(_WIN32) && !defined(__ANDROID__) && !defined(_NEWLIB_VERSION)
 #define _LIBCPP_HAS_CATOPEN 1
 #endif
+#endif
 
 #ifdef __FreeBSD__
 #define _DECLARE_C99_LDBL_MATH 1