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/unistd.h

@@ -0,0 +1,36 @@
+/*
+ * Public domain
+ * unistd.h compatibility shim
+ */
+
+#ifndef LIBCRYPTOCOMPAT_UNISTD_H
+#define LIBCRYPTOCOMPAT_UNISTD_H
+
+#ifndef _MSC_VER
+#include_next <unistd.h>
+#else
+
+#include <stdlib.h>
+#include <io.h>
+#include <process.h>
+
+#define R_OK    4
+#define W_OK    2
+#define X_OK    0
+#define F_OK    0
+
+#define access _access
+
+static inline unsigned int sleep(unsigned int seconds)
+{
+	Sleep(seconds * 1000);
+	return seconds;
+}
+
+#endif
+
+#ifndef HAVE_GETENTROPY
+int getentropy(void *buf, size_t buflen);
+#endif
+
+#endif