SCons: Add fast_unsafe option for faster rebuilds

This reverts godotengine#53828 which had caused a significant drop in incremental
rebuild time for debug builds (from 10s to 23s on my laptop).

The "faster but unsafe" options are re-added, as well as adding
`max_drift=60` which we didn't use previously.

These options speed up SCons' own processing of the codebase to decide
what to build/rebuild (i.e. the first step before actually calling the
compiler). This will therefore not make much difference for scratch
builds, and is mostly useful for incremental rebuilds (including "null"
rebuilds with no change).

These options are enabled automatically for `debug` builds, unless
`fast_unsafe=no` is passed.
They are disabled by default for `release` and `release_debug` builds,
unless `fast_unsafe=yes` is passed.

SConstruct

@@ -179,9 +179,10 @@ opts.Add(BoolVariable("use_volk", "Use the volk library to load the Vulkan loade
 
 # Advanced options
 opts.Add(BoolVariable("dev", "If yes, alias for verbose=yes warnings=extra werror=yes", False))
-opts.Add(BoolVariable("progress", "Show a progress indicator during compilation", True))
 opts.Add(BoolVariable("tests", "Build the unit tests", False))
+opts.Add(BoolVariable("fast_unsafe", "Enable unsafe options for faster rebuilds", False))
 opts.Add(BoolVariable("verbose", "Enable verbose output for the compilation", False))
+opts.Add(BoolVariable("progress", "Show a progress indicator during compilation", True))
 opts.Add(EnumVariable("warnings", "Level of compilation warnings", "all", ("extra", "all", "moderate", "no")))
 opts.Add(BoolVariable("werror", "Treat compiler warnings as errors", False))
 opts.Add("extra_suffix", "Custom extra suffix added to the base filename of all generated binary files", "")
@@ -360,6 +361,17 @@ if env_base["target"] == "debug":
     # working on the engine itself.
     env_base.Append(CPPDEFINES=["DEV_ENABLED"])
 
+# 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
+# enabled by default for `debug` builds, and can be overridden from command line.
+# Ref: https://github.com/SCons/scons/wiki/GoFastButton
+if methods.get_cmdline_bool("fast_unsafe", env_base["target"] == "debug"):
+    # Renamed to `content-timestamp` in SCons >= 4.2, keeping MD5 for compat.
+    env_base.Decider("MD5-timestamp")
+    env_base.SetOption("implicit_cache", 1)
+    env_base.SetOption("max_drift", 60)
+
 if env_base["use_precise_math_checks"]:
     env_base.Append(CPPDEFINES=["PRECISE_MATH_CHECKS"])