Do not use C++11 features yet

According to https://gcc.gnu.org/projects/cxx0x.html, the switch
-std=c++11 activates some features, that are still experimental in gcc 4.8.
Versions before 4.7 do not even know this switch.

In the moment, C++11 features should not be used for the Civetweb 1.7
release.

See also pull request #87 (use NULL instead of nullptr in order to
avoid having to add -std=c++11 to the Makefile.

src/CivetServer.cpp

@@ -98,7 +98,7 @@ CivetServer::CivetServer(const char **options,
     }
     callbacks.connection_close = closeHandler;
     context = mg_start(&callbacks, this, options);
-    if (context == nullptr) throw CivetException("null context when constructing CivetServer. Possible problem binding to port.");
+    if (context == NULL) throw CivetException("null context when constructing CivetServer. Possible problem binding to port.");
 }
 
 CivetServer::~CivetServer()