Implement S_ISDIR()

Summary: It's needed downstream in HHVM.

Reviewed By: yfeldblum

Differential Revision: D4785204

fbshipit-source-id: fe81ed8e23497fffeb44b2a366c457b788bcaada

folly/portability/SysStat.h

@@ -35,6 +35,8 @@
 #define S_IRWXU (S_IRUSR | S_IWUSR | S_IXUSR)
 #define S_IRWXG (S_IRGRP | S_IWGRP | S_IXGRP)
 
+#define S_ISDIR(mode) (((mode) & _S_IFDIR) == _S_IFDIR ? 1 : 0)
+
 // This isn't defined anywhere, so give a sane value.
 #define MAXSYMLINKS 255