BABuild.h.in: add BA Python package directory 'pythonPackDir'

cmake/configurables/BABuild.h.in

@@ -31,6 +31,10 @@ inline std::string buildBinDir() {
 inline std::string srcDir() {
     return "@CMAKE_SOURCE_DIR@";
 }
+inline std::string pythonPackDir() {
+    return "@BA_PY_SOURCE_OUTPUT_DIR@";
+}
+
 
 //! Methods to access info about Python used during the build.