git-compat-util.h: some platforms with mmap() lack MAP_FAILED definition

Some platforms with mmap() lack MAP_FAILED definition.

Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -217,7 +217,6 @@ static inline const char *skip_prefix(const char *str, const char *prefix)
 #define PROT_READ 1
 #define PROT_WRITE 2
 #define MAP_PRIVATE 1
-#define MAP_FAILED ((void*)-1)
 #endif
 
 #define mmap git_mmap
@@ -246,6 +245,10 @@ extern int git_munmap(void *start, size_t length);
 
 #endif /* NO_MMAP */
 
+#ifndef MAP_FAILED
+#define MAP_FAILED ((void *)-1)
+#endif
+
 #ifdef NO_ST_BLOCKS_IN_STRUCT_STAT
 #define on_disk_bytes(st) ((st).st_size)
 #else