Index: src/thread.cpp
===================================================================
--- src/thread.cpp
+++ src/thread.cpp
@@ -19,7 +19,7 @@
 
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 # include <sys/param.h>
-# if defined(BSD)
+# if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || defined(__DragonFly__) || defined(__APPLE__)
 #   include <sys/sysctl.h>
 # endif // defined(BSD)
 #endif // defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))