add initial CMake and Visual Studio build support

This moves the compatibility include files from include to
include/compat so we can use the awful MS C compiler
<../include/> trick to emulate the GNU #include_next extension.

This also removes a few old compat files we do not need anymore.

include/compat/netinet/in.h

@@ -0,0 +1,10 @@
+/*
+ * Public domain
+ * netinet/in.h compatibility shim
+ */
+
+#ifndef _WIN32
+#include_next <netinet/in.h>
+#else
+#include <win32netcompat.h>
+#endif

include/compat/netinet/tcp.h

@@ -0,0 +1,10 @@
+/*
+ * Public domain
+ * netinet/tcp.h compatibility shim
+ */
+
+#ifndef _WIN32
+#include_next <netinet/tcp.h>
+#else
+#include <win32netcompat.h>
+#endif