Matthew Dempsky: POSIX defines that the _POSIX_C_SOURCE macros are to be set by user
code to specify what version of POSIX the system should provide.  If
you want to check what version of POSIX is actually available, you're
supposed to test _POSIX_VERSION.

However, since sysconf() has been in POSIX since 1995, it's probably
safe to assume it's available on any system with a C++11 compiler,
especially if _SC_NPROCESSORS_ONLN is defined too.  So no point in a
complicated preprocessor rule if just we unconditionally include
<unistd.h> (on non-Windows systems).

Also, I've added a #warning for to help porters detect when a suitable
implementation isn't detected at compile-time.

Howard:  Matthew, can you patch CREDITS.TXT?  Thanks.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185275 91177308-0d34-0410-b5e6-96231b3b80d8