diff --git a/compat/posix.h b/compat/posix.h
index faaae1b655..deefc43f28 100644
--- a/compat/posix.h
+++ b/compat/posix.h
@@ -4,22 +4,33 @@
 #define _FILE_OFFSET_BITS 64
 
 /*
- * Derived from Linux "Features Test Macro" header
- * Convenience macros to test the versions of gcc (or
- * a compatible compiler).
+ * Convenience macros to test the versions of GCC (or a compatible compiler).
  * Use them like this:
  *  #if GIT_GNUC_PREREQ (2,8)
- *   ... code requiring gcc 2.8 or later ...
+ *   ... code requiring GCC 2.8 or later ...
  *  #endif
  *
+ * Note that Clang and other compilers define __GNUC__ for compatibility; use
+ * GIT_CLANG_PREREQ() to check for specific Clang versions.
+ *
  * This macro of course is not part of POSIX, but we need it for the UNUSED
  * macro which is used by some of our POSIX compatibility wrappers.
-*/
+ */
 #if defined(__GNUC__) && defined(__GNUC_MINOR__)
 # define GIT_GNUC_PREREQ(maj, min) \
-	((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
+	((__GNUC__ > (maj)) || \
+	 (__GNUC__ == (maj) && __GNUC_MINOR__ >= (min)))
 #else
- #define GIT_GNUC_PREREQ(maj, min) 0
+# define GIT_GNUC_PREREQ(maj, min) 0
+#endif
+
+/* Similar for Clang. */
+#if defined(__clang__) && defined(__clang_minor__) && defined(__clang_major__)
+# define GIT_CLANG_PREREQ(maj, min) \
+	((__clang_major__ > (maj)) || \
+	 (__clang_major__ == (maj) && __clang_minor__ >= (min)))
+#else
+# define GIT_CLANG_PREREQ(maj, min) 0
 #endif
 
 /*
@@ -35,7 +46,7 @@
  * When a parameter may be used or unused, depending on conditional
  * compilation, consider using MAYBE_UNUSED instead.
  */
-#if GIT_GNUC_PREREQ(4, 5)
+#if GIT_GNUC_PREREQ(4, 5) || GIT_CLANG_PREREQ(2, 9)
 #define UNUSED __attribute__((unused)) \
 	__attribute__((deprecated ("parameter declared as UNUSED")))
 #elif defined(__GNUC__)