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/param.h

@@ -0,0 +1,15 @@
+/*
+ * Public domain
+ * sys/param.h compatibility shim
+ */
+
+#ifndef LIBCRYPTOCOMPAT_SYS_PARAM_H
+#define LIBCRYPTOCOMPAT_SYS_PARAM_H
+
+#ifdef _MSC_VER
+#include <winsock2.h>
+#else
+#include_next <sys/param.h>
+#endif
+
+#endif