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/err.h

@@ -0,0 +1,33 @@
+/*
+ * Public domain
+ * err.h compatibility shim
+ */
+
+#ifdef HAVE_ERR_H
+
+#include_next <err.h>
+
+#else
+
+#ifndef LIBCRYPTOCOMPAT_ERR_H
+#define LIBCRYPTOCOMPAT_ERR_H
+
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+
+#define err(exitcode, format, ...) \
+  errx(exitcode, format ": %s", __VA_ARGS__, strerror(errno))
+
+#define errx(exitcode, format, ...) \
+  do { warnx(format, __VA_ARGS__); exit(exitcode); } while (0)
+
+#define warn(format, ...) \
+  warnx(format ": %s", __VA_ARGS__, strerror(errno))
+
+#define warnx(format, ...) \
+  fprintf(stderr, format "\n", __VA_ARGS__)
+
+#endif
+
+#endif