[SCons] Refactor targets, symbols, optimizations.

Now matches Godot `master` target names and supports the same flags with
the following notable exceptions:
- The default target is "template_debug", since it's compatible with
  editor builds (and TOOLS_ENABLED is never used internally).
- separate_debug_symbols is still not supported, and will be done in a
  separate commit.

.gitignore

@@ -8,6 +8,9 @@
 include/gen
 src/gen
 
+# Build configuarion.
+/custom.py
+
 # Misc
 logs/*
 *.log