Set up scons build system

- In its current state, it produces a valid, launchable Mac application package, though one that's not redistributable (relies on system-installed libraries)
- Partial support is already in-place for a Windows build

src/dialogxml/SConscript

@@ -0,0 +1,6 @@
+
+Import("env")
+
+dlog_util = env.StaticLibrary("#build/lib/dlogutil", Glob("*.cpp") + Glob("xml-parser/*.cpp"))
+
+Return("dlog_util")