SCons: Improve registration of compilation_db tool, check version

There's a builtin `toolpath` option we can use for that, so no need to hack
around a custom `scons_site` path.

The script requires SCons 3.1.1 or later, so we enable it conditionally.

Follow-up to godotengine#32848.

(cherry picked from commit 22c718a)

SConstruct

@@ -258,14 +258,13 @@ if selected_platform in platform_list:
     else:
         env = env_base.Clone()
 
-    # Custom tools are loaded automatically by SCons from site_scons/site_tools,
-    # but we want to use a different folder, so we register it manually.
-    from SCons.Script.Main import _load_site_scons_dir
+    # Compilation DB requires SCons 3.1.1+.
+    from SCons import __version__ as scons_raw_version
 
-    _load_site_scons_dir(".", "misc/scons")
-
-    env.Tool("compilation_db")
-    env.Alias("compiledb", env.CompilationDatabase("compile_commands.json"))
+    scons_ver = env._get_major_minor_revision(scons_raw_version)
+    if scons_ver >= (3, 1, 1):
+        env.Tool("compilation_db", toolpath=["misc/scons"])
+        env.Alias("compiledb", env.CompilationDatabase("compile_commands.json"))
 
     if env['dev']:
         env['verbose'] = True