Only link C++ libraries by default when run via em++ or when compiling a C++ file.

[sbc100]

if you try to link C++ object files with clang or gcc (as opposed to clang++ or g++) the result is undefined C++ symbols.

We should behave the same. Up until #10998
this was a non-issue since we were doing "on-demand" linking for all libraries.

This risks breaking certain users, so it might not be worth it.

The main upside would be that it would reduce the size of C programs build with MAIN_MODULE=1 .. (and be more consistent with existing toolchains).

[stale]

This issue has been automatically marked as stale because there has been no activity in the past year. It will be closed automatically if no further activity occurs in the next 30 days. Feel free to re-open at any time if this issue is still relevant.

[sbc100]

Still working on this