Add FreeBSD support

Summary:
This set of changes makes it possible to compile `flow` on FreeBSD.

It does require `libinotify` to be install separately, but otherwise seems fine.  I'm not sure how to make the local `libinotify` a dependency through the `opam` build system.

Additionally, the current install instructions in `README.md` specify that `opam init --comp 4.03.0` is sufficient to change versions but on FreeBSD it is necessary to use `opam switch 4.03.0` since the `init` subcommand will not replace the system version (currently 4.02.3 on FreeBSD) which is automatically installed as a dependency of `opam`.
Closes facebook/flow#4433

Reviewed By: samwgoldman

Differential Revision: D5502390

Pulled By: gabelevi

fbshipit-source-id: 06c2310702c6f34980849776cdbe2fb72542baa1

hphp/hack/src/heap/hh_shared.c

@@ -148,7 +148,7 @@
  * with the MAP_ANONYMOUS flag. The memfd_create() system call first
  * appeared in Linux 3.17.
  ****************************************************************************/
-#if !defined __APPLE__ && !defined _WIN32
+#ifdef __linux__
   // Linux version for the architecture must support syscall memfd_create
   #ifndef SYS_memfd_create
     #if defined(__x86_64__)
@@ -176,6 +176,11 @@
   }
 #endif
 
+#ifndef MAP_NORESERVE
+  // This flag was unimplemented in FreeBSD and then later removed
+  #define MAP_NORESERVE 0
+#endif
+
 // The following 'typedef' won't be required anymore
 // when dropping support for OCaml < 4.03
 #ifdef __MINGW64__

hphp/hack/src/third-party/inotify/inotify_stubs.c

@@ -1,5 +1,5 @@
 /*
- *	Copyright (C) 2006-2008 Vincent Hanquez <vincent@snarc.org>
+ * Copyright (C) 2006-2008 Vincent Hanquez <vincent@snarc.org>
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU Lesser General Public License as published
@@ -28,7 +28,6 @@
 #include <caml/callback.h>
 #include <caml/unixsupport.h>
 
-#include <features.h>
 #include <sys/inotify.h>
 
 static int inotify_flag_table[] = {

hphp/hack/src/utils/sysinfo.c

@@ -13,11 +13,9 @@
 #include <caml/mlvalues.h>
 
 #include <assert.h>
-#ifndef _WIN32
-#ifndef __APPLE__
+#ifdef __linux__
 #include <sys/sysinfo.h>
 #endif
-#endif
 
 #ifdef _WIN32
 #include <windows.h>