SCons: Disable C++ exception handling

Upon investigating the extremely slow MSVC build times in #80513, I noticed
that while Godot policy is to never use exceptions, we weren't enforcing it
with compiler flags, and thus still included exception handling code and
stack unwinding.

This is wasteful on multiple aspects:

- Binary size: Around 20% binary size reduction with exceptions disabled
  for both MSVC and GCC binaries.
- Compile time:
  * More than 50% build time reduction with MSVC.
  * 10% to 25% build time reduction with GCC + LTO.
- Performance: Possibly, needs to be benchmarked.

Since users may want to re-enable exceptions in their own thirdparty code
or the libraries they compile with Godot, this behavior can be toggled with
the `disable_exceptions` SCons option, which defaults to true.

2 files changed, 12 insertions, 2 deletions

diff --git a/modules/denoise/SCsub b/modules/denoise/SCsub
index 779ce165d2..967a511e1e 100644
--- a/modules/denoise/SCsub
+++ b/modules/denoise/SCsub
@@ -109,6 +109,15 @@ env_oidn.AppendUnique(CPPDEFINES=["NDEBUG"])  # No assert() even in debug builds
 
 env_thirdparty = env_oidn.Clone()
 env_thirdparty.disable_warnings()
+
+if env["disable_exceptions"]:
+    # OIDN hard-requires exceptions, so we re-enable them here.
+    if env.msvc and ("_HAS_EXCEPTIONS", 0) in env_thirdparty["CPPDEFINES"]:
+        env_thirdparty["CPPDEFINES"].remove(("_HAS_EXCEPTIONS", 0))
+        env_thirdparty.AppendUnique(CCFLAGS=["/EHsc"])
+    elif not env.msvc and "-fno-exceptions" in env_thirdparty["CCFLAGS"]:
+        env_thirdparty["CCFLAGS"].remove("-fno-exceptions")
+
 env_thirdparty.add_source_files(thirdparty_obj, thirdparty_sources)
 env.modules_sources += thirdparty_obj
 
diff --git a/modules/openxr/SCsub b/modules/openxr/SCsub
index f49dc390de..1bd10f1009 100644
--- a/modules/openxr/SCsub
+++ b/modules/openxr/SCsub
@@ -52,10 +52,11 @@ if env["builtin_openxr"]:
 
     env_thirdparty = env_openxr.Clone()
     env_thirdparty.disable_warnings()
+
     env_thirdparty.AppendUnique(CPPDEFINES=["DISABLE_STD_FILESYSTEM"])
+    if env["disable_exceptions"]:
+        env_thirdparty.AppendUnique(CPPDEFINES=["XRLOADER_DISABLE_EXCEPTION_HANDLING", ("JSON_USE_EXCEPTION", 0)])
 
-    if "-fno-exceptions" in env_thirdparty["CXXFLAGS"]:
-        env_thirdparty["CXXFLAGS"].remove("-fno-exceptions")
     env_thirdparty.Append(CPPPATH=[thirdparty_dir + "/src/loader"])
 
     # add in external jsoncpp dependency