Jolt: Fix multiple definition LTO linking issue with mingw-gcc

[akien-mga]

This fixes the following errors when building with mingw-gcc for Windows with scons p=windows lto=full:

/usr/lib/gcc/x86_64-w64-mingw32/14.1.1/../../../../x86_64-w64-mingw32/bin/ld: modules/jolt_physics/spaces/jolt_layers.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD1Ev[_ZThn16_N10JoltLayersD1Ev]+0x0): multiple definition of `JoltLayers::~JoltLayers()'; modules/jolt_physics/spaces/jolt_space_3d.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD1Ev[_ZThn8_N10JoltLayersD1Ev]+0x0): first defined here
/usr/lib/gcc/x86_64-w64-mingw32/14.1.1/../../../../x86_64-w64-mingw32/bin/ld: modules/jolt_physics/spaces/jolt_layers.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD1Ev[_ZThn16_N10JoltLayersD1Ev]+0x0): multiple definition of `non-virtual thunk to JoltLayers::~JoltLayers()'; modules/jolt_physics/spaces/jolt_space_3d.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD1Ev[_ZThn8_N10JoltLayersD1Ev]+0x0): first defined here
/usr/lib/gcc/x86_64-w64-mingw32/14.1.1/../../../../x86_64-w64-mingw32/bin/ld: modules/jolt_physics/spaces/jolt_layers.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD1Ev[_ZThn16_N10JoltLayersD1Ev]+0x0): multiple definition of `non-virtual thunk to JoltLayers::~JoltLayers()'; modules/jolt_physics/spaces/jolt_space_3d.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD1Ev[_ZThn8_N10JoltLayersD1Ev]+0x0): first defined here
/usr/lib/gcc/x86_64-w64-mingw32/14.1.1/../../../../x86_64-w64-mingw32/bin/ld: modules/jolt_physics/spaces/jolt_layers.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD0Ev[_ZThn16_N10JoltLayersD0Ev]+0x0): multiple definition of `JoltLayers::~JoltLayers()'; modules/jolt_physics/spaces/jolt_space_3d.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD0Ev[_ZThn8_N10JoltLayersD0Ev]+0x0): first defined here
/usr/lib/gcc/x86_64-w64-mingw32/14.1.1/../../../../x86_64-w64-mingw32/bin/ld: modules/jolt_physics/spaces/jolt_layers.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD0Ev[_ZThn16_N10JoltLayersD0Ev]+0x0): multiple definition of `non-virtual thunk to JoltLayers::~JoltLayers()'; modules/jolt_physics/spaces/jolt_space_3d.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD0Ev[_ZThn8_N10JoltLayersD0Ev]+0x0): first defined here
/usr/lib/gcc/x86_64-w64-mingw32/14.1.1/../../../../x86_64-w64-mingw32/bin/ld: modules/jolt_physics/spaces/jolt_layers.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD0Ev[_ZThn16_N10JoltLayersD0Ev]+0x0): multiple definition of `non-virtual thunk to JoltLayers::~JoltLayers()'; modules/jolt_physics/spaces/jolt_space_3d.windows.editor.x86_64.o (symbol from plugin):(.gnu.linkonce.t._ZN10JoltLayersD0Ev[_ZThn8_N10JoltLayersD0Ev]+0x0): first defined here

@hpvb gave me this advice:

This means that during the LTO step we end up with multiple symbols of the same name, probably means something has to be moved out of a header to a .cpp file

And indeed defining the empty destructor in the .cpp file solved it. Previous attempts at adding ~JoltLayers(); or ~JoltLayers = default; in the header wouldn't work.

For the record, this sounds related to this GCC bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94156
But it's unconfirmed upstream so I don't know if it's indeed a GCC bug or just a spec caveat.

[hpvb]

Yeah, I'm also not entirely clear if this is a bug in the linker, or just something that only works when you don't use LTO by accident. Maybe without LTO these duplicate symbols get folded together by the linker but with LTO not? I really don't know.

[Repiteo]

Thanks!