updated for version 7.4.055
Problem:    Mac: Where availability macros are defined depends on the system.
Solution:   Add a configure check. (Felix Bünemann)
diff --git a/src/os_mac.h b/src/os_mac.h
index 78b79c2..c13a301 100644
--- a/src/os_mac.h
+++ b/src/os_mac.h
@@ -16,6 +16,11 @@
 # define OPAQUE_TOOLBOX_STRUCTS 0
 #endif
 
+/* Include MAC_OS_X_VERSION_* macros */
+#ifdef HAVE_AVAILABILITYMACROS_H
+# include <AvailabilityMacros.h>
+#endif
+
 /*
  * Macintosh machine-dependent things.
  *
@@ -263,7 +268,7 @@
 #endif
 
 /* Some "prep work" definition to be able to compile the MacOS X
- * version with os_unix.x instead of os_mac.c. Based on the result
+ * version with os_unix.c instead of os_mac.c. Based on the result
  * of ./configure for console MacOS X.
  */