SCU build - Change options to "yes / no"

To save confusion for users who prefer yes / no, rather than none / dev / all.

SConstruct

@@ -224,7 +224,7 @@ opts.Add(
     "",
 )
 opts.Add(BoolVariable("use_precise_math_checks", "Math checks use very precise epsilon (debug option)", False))
-opts.Add(EnumVariable("scu_build", "Use single compilation unit build", "none", ("none", "dev", "all")))
+opts.Add(BoolVariable("scu_build", "Use single compilation unit build", False))
 
 # Thirdparty libraries
 opts.Add(BoolVariable("builtin_certs", "Use the built-in SSL certificates bundles", True))
@@ -430,20 +430,14 @@ if env_base.debug_features:
     # to give *users* extra debugging information for their game development.
     env_base.Append(CPPDEFINES=["DEBUG_ENABLED"])
 
-
 if env_base.dev_build:
     # DEV_ENABLED enables *engine developer* code which should only be compiled for those
     # working on the engine itself.
     env_base.Append(CPPDEFINES=["DEV_ENABLED"])
-    env_base["use_scu"] = env_base["scu_build"] in ("dev", "all")
 else:
     # Disable assert() for production targets (only used in thirdparty code).
     env_base.Append(CPPDEFINES=["NDEBUG"])
 
-    # SCU builds currently use a lot of compiler memory
-    # in release builds, so disallow outside of DEV builds unless "all" is set.
-    env_base["use_scu"] = env_base["scu_build"] == "all"
-
 # SCons speed optimization controlled by the `fast_unsafe` option, which provide
 # more than 10 s speed up for incremental rebuilds.
 # Unsafe as they reduce the certainty of rebuilding all changed files, so it's
@@ -559,7 +553,7 @@ if selected_platform in platform_list:
         env["lto"] = ARGUMENTS.get("lto", "auto")
 
     # Run SCU file generation script if in a SCU build.
-    if env["use_scu"]:
+    if env["scu_build"]:
         methods.set_scu_folders(scu_builders.generate_scu_files(env["verbose"], env_base.dev_build == False))
 
     # Must happen after the flags' definition, as configure is when most flags

methods.py

@@ -79,7 +79,7 @@ def _find_scu_section_name(subdir):
 
 
 def add_source_files_scu(self, sources, files, allow_gen=False):
-    if self["use_scu"] and isinstance(files, str):
+    if self["scu_build"] and isinstance(files, str):
         if "*." not in files:
             return False