[libc++] Use __has_include instead of complex logic in thread.cpp

We might end up including more headers than strictly necessary this way,
but it's much simpler and it makes it easier to port thread.cpp to systems
not handled by the existing conditionals.

diff --git a/libcxx/src/thread.cpp b/libcxx/src/thread.cpp
index 5f44e9e..e1dc972 100644 (file)
--- a/libcxx/src/thread.cpp
+++ b/libcxx/src/thread.cpp
@@ -14,17 +14,21 @@
 #include "vector"
 #include "future"
 #include "limits"
-#include <sys/types.h>
 
-#if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
+#if __has_include(<sys/types.h>)
+# include <sys/types.h>
+#endif
+
+#if __has_include(<sys/param.h>)
 # include <sys/param.h>
-# if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || defined(__DragonFly__) || defined(__APPLE__)
-#   include <sys/sysctl.h>
-# endif
-#endif // defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
+#endif
+
+#if __has_include(<sys/sysctl.h>)
+# include <sys/sysctl.h>
+#endif
 
 #if __has_include(<unistd.h>)
-#include <unistd.h>
+# include <unistd.h>
 #endif
 
 #if defined(__NetBSD__)