Reduce template instantiations of std::vector<std::string>

The new StringMap code for luax_enumerror causes code to be generated for
std::vector. By using an extern template (an old feature standardised in c++11)
we can instantiate it once instead, in StringMap.cpp.

--HG--
branch : minor

src/common/StringMap.cpp

@@ -0,0 +1,25 @@
+/**
+ * Copyright (c) 2006-2017 LOVE Development Team
+ *
+ * This software is provided 'as-is', without any express or implied
+ * warranty.  In no event will the authors be held liable for any damages
+ * arising from the use of this software.
+ *
+ * Permission is granted to anyone to use this software for any purpose,
+ * including commercial applications, and to alter it and redistribute it
+ * freely, subject to the following restrictions:
+ *
+ * 1. The origin of this software must not be misrepresented; you must not
+ *    claim that you wrote the original software. If you use this software
+ *    in a product, an acknowledgment in the product documentation would be
+ *    appreciated but is not required.
+ * 2. Altered source versions must be plainly marked as such, and must not be
+ *    misrepresented as being the original software.
+ * 3. This notice may not be removed or altered from any source distribution.
+ **/
+
+#include "StringMap.h"
+
+// See the header
+template class std::vector<std::string>;
+template decltype(std::vector<std::string>().emplace_back("")) std::vector<std::string>::emplace_back<const char *const&>(const char *const&);