Always #define _GNU_SOURCE to pull in pipe2() declarations

Summary: [folly::Subprocess] Always #define _GNU_SOURCE to pull in pipe2() declarations

Test Plan: Compile

Reviewed By: tudorb@fb.com

FB internal diff: D1327004

folly/Subprocess.cpp

@@ -21,6 +21,10 @@
 #endif
 #include <fcntl.h>
 #include <poll.h>
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
 #include <unistd.h>
 
 #include <array>