Add a local implementation of hstrerror for the system which do not have it

The function converts the value of h_errno (last error of name
resolver library, see netdb.h).
One of systems which supposedly do not have the function is SunOS.
POSIX does not mandate its presence.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -167,6 +167,11 @@ extern size_t gitstrlcpy(char *, const char *, size_t);
 extern uintmax_t gitstrtoumax(const char *, char **, int);
 #endif
 
+#ifdef NO_HSTRERROR
+#define hstrerror githstrerror
+extern const char *githstrerror(int herror);
+#endif
+
 extern void release_pack_memory(size_t, int);
 
 static inline char* xstrdup(const char *str)