[BL] Add explicit ptree instantiation.

This reduces compile time for multiple translation units.

BaseLib/ConfigTree.cpp

@@ -14,6 +14,11 @@
 
 #include <logog/include/logog.hpp>
 
+// Explicitly instantiate the boost::property_tree::ptree which is a typedef to
+// the following basic_ptree.
+template class boost::property_tree::basic_ptree<std::string, std::string,
+                                                 std::less<std::string>>;
+
 namespace BaseLib
 {
 

BaseLib/ConfigTree.h

@@ -10,9 +10,13 @@
 #ifndef BASELIB_CONFIGTREE_H_
 #define BASELIB_CONFIGTREE_H_
 
+#include <string>
 #include <boost/property_tree/ptree.hpp>
 #include <boost/filesystem.hpp>
 
+extern template class boost::property_tree::basic_ptree<
+    std::string, std::string, std::less<std::string>>;
+
 namespace BaseLib
 {
 using ConfigTree = boost::property_tree::ptree;