base: Fix incorrect CPP guard

This was guarded on `darwin_HOST_OS` instead of `defined(darwin_HOST_OS)`.

libraries/base/include/HsBase.h

@@ -540,7 +540,7 @@ INLINE int __hscore_open(char *file, int how, mode_t mode) {
 }
 #endif
 
-#if darwin_HOST_OS
+#if defined(darwin_HOST_OS)
 // You should not access _environ directly on Darwin in a bundle/shared library.
 // See #2458 and http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man7/environ.7.html
 #include <crt_externs.h>