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/uio.h

@@ -0,0 +1,17 @@
+/*
+ * Public domain
+ * sys/select.h compatibility shim
+ */
+
+#ifndef _WIN32
+#include_next <sys/uio.h>
+#else
+
+#include <sys/types.h>
+
+struct iovec {
+	void *iov_base;
+	size_t iov_len;
+};
+
+#endif