Pass -c when compiling SDL2 object files

[sbc100]

Without -c emscripten will try to run lld on the object files its
given to produce another object file. We should probably disable that
for the single build case. This breakage was introduced in #9444 and
we should probably address it, but compiling without -c is also
broken so fixing here.

Fixes #9510

[kripken]

Without -c emscripten will try to run lld on the object files its given to produce another object file.

That surprises me - I didn't realize that was happening when looking at the recent linking PRs. I think we should fix that directly, i.e., emcc a.c -o a.o does the same thing even if adding -c, as it always did.

I realize it's a little odd that emscripten looks at the suffix of the output file to know what to emit, and I'm open to finding a path to changing that, but we need to think carefully about how to do that without breaking users.

[kripken]

Alternatively, maybe we should make emcc a.c -o a.o error if it doesn't have -c provided? I think that would move things in the direction you want @sbc100 ? That would break people but at least a clear error would make it an easy fix probably.

[kripken]

Regardless of the linking regression fix this PR looks good.

[sbc100]

Without -c emscripten will try to run lld on the object files its given to produce another object file.

That surprises me - I didn't realize that was happening when looking at the recent linking PRs. I think we should fix that directly, i.e., emcc a.c -o a.o does the same thing even if adding -c, as it always did.

I realize it's a little odd that emscripten looks at the suffix of the output file to know what to emit, and I'm open to finding a path to changing that, but we need to think carefully about how to do that without breaking users.

I agree and I will restore the behavior. Its a little more subtle than one might imagine though.

The old behaviour had a special case for a single object file:

emcc test1.c test2.c -o test.o -> Compile each source file then use lld to link them together into a bigger object file.

emcc test1.c -o test.o -> Compile the single source file then skip using lld since its already a single object and just copy it.

The new behaviour is that it run lld in all cases, even on a single object file. This means that if you pass certain flags such as -s RELOCATABLE you might get some very different behaviour.

The SDL build script was missing the -c but due to the previous "just copy it if its a single object" file this was completely ignored/harmless.

I'm happy to temporarily restore the previous behaviour.

However, in the long run I think we can do a bit better. I don't think either of the above use cases is useful or worth maintaining. There is a use case for linking object into a single object, but I think its rare and we should only allow it when all the input files are also objects. So passing a source file with output as an object file without -c would be a hard error.