SERVER-8994: Boost 1.55 MacOS X fixes

1. Move swap from std to mongo namespace. Fixed C++ build on 10.9 with
XCode 5.1
2. Add check to see if no-null-conversion needs to be enabled for C++
build on 10.8 with Xcode 4.2

SConstruct

@@ -1165,6 +1165,31 @@ def doConfigure(myenv):
         # primary mongo sources as well.
         AddToCCFLAGSIfSupported(myenv, "-Wno-unused-const-variable")
 
+    # Check if we need to disable null-conversion warnings
+    if using_clang():
+        def CheckNullConversion(context):
+
+            test_body = """
+            #include <boost/shared_ptr.hpp>
+            struct TestType { int value; bool boolValue; };
+            bool foo() {
+                boost::shared_ptr<TestType> sp(new TestType);
+                return NULL != sp;
+            }
+            """
+
+            context.Message('Checking if implicit boost::shared_ptr null conversion is supported... ')
+            ret = context.TryCompile(textwrap.dedent(test_body), ".cpp")
+            context.Result(ret)
+            return ret
+
+        conf = Configure(myenv, help=False, custom_tests = {
+            'CheckNullConversion' : CheckNullConversion,
+        })
+        if conf.CheckNullConversion() == False:
+            env.Append( CCFLAGS="-Wno-null-conversion" )
+        conf.Finish()
+
     # This needs to happen before we check for libc++, since it affects whether libc++ is available.
     if darwin and has_option('osx-version-min'):
         min_version = get_option('osx-version-min')

src/mongo/db/pipeline/document.h

@@ -544,16 +544,10 @@ namespace mongo {
         std::vector<Value> _array;
     };
 
-}
-
-namespace std {
-    template<>
     inline void swap(mongo::Document& lhs, mongo::Document& rhs) { lhs.swap(rhs); }
-}
 
 /* ======================= INLINED IMPLEMENTATIONS ========================== */
 
-namespace mongo {
     inline FieldIterator Document::fieldIterator() const {
         return FieldIterator(*this);
     }