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/string.h

@@ -0,0 +1,82 @@
+/*
+ * Public domain
+ * string.h compatibility shim
+ */
+
+#ifndef LIBCRYPTOCOMPAT_STRING_H
+#define LIBCRYPTOCOMPAT_STRING_H
+
+#ifdef _MSC_VER
+#include <../include/string.h>
+#else
+#include_next <string.h>
+#endif
+
+#include <sys/types.h>
+
+#if defined(__sun) || defined(__hpux)
+/* Some functions historically defined in string.h were placed in strings.h by
+ * SUS. Use the same hack as OS X and FreeBSD use to work around on Solaris and HPUX.
+ */
+#include <strings.h>
+#endif
+
+#ifndef HAVE_STRCASECMP
+int strcasecmp(const char *s1, const char *s2);
+int strncasecmp(const char *s1, const char *s2, size_t len);
+#endif
+
+#ifndef HAVE_STRLCPY
+size_t strlcpy(char *dst, const char *src, size_t siz);
+#endif
+
+#ifndef HAVE_STRLCAT
+size_t strlcat(char *dst, const char *src, size_t siz);
+#endif
+
+#ifndef HAVE_STRNDUP
+char * strndup(const char *str, size_t maxlen);
+/* the only user of strnlen is strndup, so only build it if needed */
+#ifndef HAVE_STRNLEN
+size_t strnlen(const char *str, size_t maxlen);
+#endif
+#endif
+
+#ifndef HAVE_STRSEP
+char *strsep(char **stringp, const char *delim);
+#endif
+
+#ifndef HAVE_EXPLICIT_BZERO
+void explicit_bzero(void *, size_t);
+#endif
+
+#ifndef HAVE_TIMINGSAFE_BCMP
+int timingsafe_bcmp(const void *b1, const void *b2, size_t n);
+#endif
+
+#ifndef HAVE_TIMINGSAFE_MEMCMP
+int timingsafe_memcmp(const void *b1, const void *b2, size_t len);
+#endif
+
+#ifndef HAVE_MEMMEM
+void * memmem(const void *big, size_t big_len, const void *little,
+	size_t little_len);
+#endif
+
+#ifdef _WIN32
+#include <errno.h>
+
+static inline char  *
+posix_strerror(int errnum)
+{
+	if (errnum == ECONNREFUSED) {
+		return "Connection refused";
+	}
+	return strerror(errnum);
+}
+
+#define strerror(errnum) posix_strerror(errnum)
+
+#endif
+
+#endif