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/time.h

@@ -0,0 +1,16 @@
+/*
+ * Public domain
+ * sys/time.h compatibility shim
+ */
+
+#ifndef LIBCRYPTOCOMPAT_TIME_H
+#define LIBCRYPTOCOMPAT_TIME_H
+
+#ifdef _MSC_VER
+#include <../include/time.h>
+#define gmtime_r(tp, tm) ((gmtime_s((tm), (tp)) == 0) ? (tm) : NULL)
+#else
+#include_next <time.h>
+#endif
+
+#endif