mac: Suppress partial availability warnings.

util/stdlib/strnlen.cc

Index: util/stdlib/strnlen.cc
diff --git a/util/stdlib/strnlen.cc b/util/stdlib/strnlen.cc
index b17796bb8e439152dfd9d37c888bb6ee1cd7554d..3016bf63efa4970fc7b7f6da964fd8659fb57faa 100644
--- a/util/stdlib/strnlen.cc
+++ b/util/stdlib/strnlen.cc
@@ -14,9 +14,18 @@
 
 #include "util/stdlib/strnlen.h"
 
+#if defined(OS_MACOSX) && MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_7
+
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_7
+// Redeclare a method only available on OSX 10.7+ to suppress a
+// -Wpartial-availability warning.
+extern "C" {
+size_t strnlen(const char* string, size_t max_length);
+}  // extern "C"
+#endif
+
 namespace crashpad {
 
-#if defined(OS_MACOSX) && MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_7
 size_t strnlen(const char* string, size_t max_length) {
 #if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_7
   if (::strnlen) {
@@ -32,6 +41,7 @@ size_t strnlen(const char* string, size_t max_length) {
 
   return max_length;
 }
-#endif
 
 }  // namespace crashpad
+
+#endif