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/dirent.h

@@ -0,0 +1,17 @@
+/*
+ * Public domain
+ * dirent.h compatibility shim
+ */
+
+#ifndef LIBCRYPTOCOMPAT_DIRENT_H
+#define LIBCRYPTOCOMPAT_DIRENT_H
+
+#ifdef _MSC_VER
+#include <windows.h>
+#include <dirent_msvc.h>
+#else
+#include_next <dirent.h>
+#endif
+
+#endif
+