Fix compilation with VS2017

The primary issue is that in MSVC 14.00 (VS2015) Microsoft added
snprintf as a function to the standard library and prevents users from
defining it to something else (typically, this was _snprintf). So, only
define it when using _MSC_VER < 1900.

Other changes are:
- Fix macro definition of dup2
- Add macro for getpid
- Add function definition for win32_dup
- Add missing EXTERNs

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

lib/init.c

@@ -22,6 +22,7 @@
 #define _GNU_SOURCE
 
 #if defined(WIN32)
+#include "win32/win32_compat.h"
 #else
 #include <strings.h>
 #include <unistd.h>