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/sys/mman.h

@@ -0,0 +1,19 @@
+/*
+ * Public domain
+ * sys/mman.h compatibility shim
+ */
+
+#include_next <sys/mman.h>
+
+#ifndef LIBCRYPTOCOMPAT_MMAN_H
+#define LIBCRYPTOCOMPAT_MMAN_H
+
+#ifndef MAP_ANON
+#ifdef MAP_ANONYMOUS
+#define MAP_ANON MAP_ANONYMOUS
+#else
+#error "System does not support mapping anonymous pages?"
+#endif
+#endif
+
+#endif