mac: Suppress partial availability warnings.

util/stdlib/strnlen.cc

 // Copyright 2014 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
 #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) {
     return ::strnlen(string, max_length);
   }
 #endif
 
   for (size_t index = 0; index < max_length; ++index) {
     if (string[index] == '\0') {
       return index;
     }
   }
 
   return max_length;
 }
-#endif
 
 }  // namespace crashpad
+
+#endif