Required since Godot 4.3, which is also the first Godot version with
wide WASM gdnative support (previous versions were Chrome-only, and very
brittle).
Was using CPPFLAGS, but should use the explicit scons CCFLAGS which
makes it clear they are applied to both the C and C++ compiler.
CPPFLAGS was also fine (they are preprocessor flags, also applied to
both C and C++), but we should try to stay consistent with what we do
in Godot.
SUPPORT_LONGJMP have changed since emscripten 3.1.32 to default to
"wasm" mode when exceptions are enabled, and "emscripten" mode when
disabled.
While we generally doesn't use exception in core, linked libraries may
need them, and emscripten don't plan to support WASM EH + Emscripten
SjLj in the long term.
Split `targets` tool logic, moving all the compiler-specific flags to a
new `common_compiler_flags.py` file, and everything else (CPPDEFINES,
optimize option logic, dev build logic, etc) to the `godotcpp` tool.
The default tools now apply the common compiler flags by importing the
file and explicitly calling `configure`.
