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.h

@@ -26,6 +26,12 @@
 #include <string>
 #include <vector>
 
+// As StringMap instantiates std::vector<std::string> for instances that use
+// getNames(), we end up with multiple copies in the object files. This
+// declaration means we only emit it once (in StringMap.cpp).
+extern template class std::vector<std::string>;
+extern template decltype(std::vector<std::string>().emplace_back("")) std::vector<std::string>::emplace_back<const char *const&>(const char *const&);
+
 namespace love
 {