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/stdlib.h

@@ -0,0 +1,34 @@
+/*
+ * stdlib.h compatibility shim
+ * Public domain
+ */
+
+#ifndef LIBCRYPTOCOMPAT_STDLIB_H
+#define LIBCRYPTOCOMPAT_STDLIB_H
+
+#ifdef _MSC_VER
+#include <../include/stdlib.h>
+#else
+#include_next <stdlib.h>
+#endif
+
+#include <sys/types.h>
+#include <sys/stat.h>
+//#include <sys/time.h>
+#include <stdint.h>
+
+#ifndef HAVE_ARC4RANDOM_BUF
+uint32_t arc4random(void);
+void arc4random_buf(void *_buf, size_t n);
+#endif
+
+#ifndef HAVE_REALLOCARRAY
+void *reallocarray(void *, size_t, size_t);
+#endif
+
+#ifndef HAVE_STRTONUM
+long long strtonum(const char *nptr, long long minval,
+		long long maxval, const char **errstr);
+#endif
+
+#endif