Add option in SCons to copy environment variables

This allows custom environment variables to be used during the build,
which is useful when using alternative compilation tools such as caches
and distributed build systems.

SConstruct

@@ -212,6 +212,7 @@ opts.Add("extra_suffix", "Custom extra suffix added to the base filename of all
 opts.Add("object_prefix", "Custom prefix added to the base filename of all generated object files", "")
 opts.Add(BoolVariable("vsproj", "Generate a Visual Studio solution", False))
 opts.Add("vsproj_name", "Name of the Visual Studio solution", "godot")
+opts.Add("import_env_vars", "A comma-separated list of environment variables to copy from the outer environment.", "")
 opts.Add(BoolVariable("disable_3d", "Disable 3D nodes for a smaller executable", False))
 opts.Add(BoolVariable("disable_advanced_gui", "Disable advanced GUI nodes and behaviors", False))
 opts.Add("build_profile", "Path to a file containing a feature build profile", "")
@@ -270,6 +271,12 @@ opts.Add("LINKFLAGS", "Custom flags for the linker")
 # in following code (especially platform and custom_modules).
 opts.Update(env_base)
 
+# Copy custom environment variables if set.
+if env_base["import_env_vars"]:
+    for env_var in str(env_base["import_env_vars"]).split(","):
+        if env_var in os.environ:
+            env_base["ENV"][env_var] = os.environ[env_var]
+
 # Platform selection: validate input, and add options.
 
 selected_platform = ""