Single Compilation Unit build.

Adds support for simple SCU build.
This speeds up compilation by compiling multiple cpp files within a single translation unit.

methods.py

@@ -13,8 +13,23 @@ from SCons.Script import ARGUMENTS
 from SCons.Script import Glob
 from SCons.Variables.BoolVariable import _text2bool
 
+from pathlib import Path
+from os.path import normpath, basename
 
-def add_source_files(self, sources, files):
+# Get the "Godot" folder name ahead of time
+base_folder_path = str(os.path.abspath(Path(__file__).parent)) + "/"
+base_folder_only = os.path.basename(os.path.normpath(base_folder_path))
+# Listing all the folders we have converted
+# for SCU in scu_builders.py
+_scu_folders = set()
+
+
+def set_scu_folders(scu_folders):
+    global _scu_folders
+    _scu_folders = scu_folders
+
+
+def add_source_files_orig(self, sources, files, allow_gen=False):
     # Convert string to list of absolute paths (including expanding wildcard)
     if isbasestring(files):
         # Keep SCons project-absolute path as they are (no wildcard support)
@@ -29,7 +44,7 @@ def add_source_files(self, sources, files):
             skip_gen_cpp = "*" in files
             dir_path = self.Dir(".").abspath
             files = sorted(glob.glob(dir_path + "/" + files))
-            if skip_gen_cpp:
+            if skip_gen_cpp and not allow_gen:
                 files = [f for f in files if not f.endswith(".gen.cpp")]
 
     # Add each path as compiled Object following environment (self) configuration
@@ -41,6 +56,71 @@ def add_source_files(self, sources, files):
         sources.append(obj)
 
 
+# The section name is used for checking
+# the hash table to see whether the folder
+# is included in the SCU build.
+# It will be something like "core/math".
+def _find_scu_section_name(subdir):
+    section_path = os.path.abspath(subdir) + "/"
+
+    folders = []
+    folder = ""
+
+    for i in range(8):
+        folder = os.path.dirname(section_path)
+        folder = os.path.basename(folder)
+        if folder == base_folder_only:
+            break
+        folders += [folder]
+        section_path += "../"
+        section_path = os.path.abspath(section_path) + "/"
+
+    section_name = ""
+    for n in range(len(folders)):
+        section_name += folders[len(folders) - n - 1]
+        if n != (len(folders) - 1):
+            section_name += "/"
+
+    return section_name
+
+
+def add_source_files_scu(self, sources, files, allow_gen=False):
+    if self["scu_build"] and isinstance(files, str):
+        if "*." not in files:
+            return False
+
+        # If the files are in a subdirectory, we want to create the scu gen
+        # files inside this subdirectory.
+        subdir = os.path.dirname(files)
+        if subdir != "":
+            subdir += "/"
+
+        section_name = _find_scu_section_name(subdir)
+        # if the section name is in the hash table?
+        # i.e. is it part of the SCU build?
+        global _scu_folders
+        if section_name not in (_scu_folders):
+            return False
+
+        if self["verbose"]:
+            print("SCU building " + section_name)
+
+        # Add all the gen.cpp files in the SCU directory
+        add_source_files_orig(self, sources, subdir + ".scu/scu_*.gen.cpp", True)
+        return True
+    return False
+
+
+# Either builds the folder using the SCU system,
+# or reverts to regular build.
+def add_source_files(self, sources, files, allow_gen=False):
+    if not add_source_files_scu(self, sources, files, allow_gen):
+        # Wraps the original function when scu build is not active.
+        add_source_files_orig(self, sources, files, allow_gen)
+        return False
+    return True
+
+
 def disable_warnings(self):
     # 'self' is the environment
     if self.msvc: