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/stdio.h

@@ -0,0 +1,39 @@
+/*
+ * Public domain
+ * stdio.h compatibility shim
+ */
+
+#ifndef LIBCRYPTOCOMPAT_STDIO_H
+#define LIBCRYPTOCOMPAT_STDIO_H
+
+#ifdef _MSC_VER
+#include <../include/stdio.h>
+#else
+#include_next <stdio.h>
+#endif
+
+#ifndef HAVE_ASPRINTF
+#include <stdarg.h>
+int vasprintf(char **str, const char *fmt, va_list ap);
+int asprintf(char **str, const char *fmt, ...);
+#endif
+
+#ifdef _WIN32
+
+void posix_perror(const char *s);
+FILE * posix_fopen(const char *path, const char *mode);
+int posix_rename(const char *oldpath, const char *newpath);
+
+#ifndef NO_REDEF_POSIX_FUNCTIONS
+#define perror(errnum) posix_perror(errnum)
+#define fopen(path, mode) posix_fopen(path, mode)
+#define rename(oldpath, newpath) posix_rename(oldpath, newpath)
+#endif
+
+#ifdef _MSC_VER
+#define snprintf _snprintf
+#endif
+
+#endif
+
+#endif