Turn off shared library signature check when configuring

[hoodmane]

AC_CHECK_LIB(somelib, func) writes the following to conftest.c:

char func ();

int main(void) {
   return func ();
}

Then runs emcc conftest.c -lsomelib and checks whether it succeeds. With static libraries the signature mismatch is just a warning but with shared libraries it is a hard error. Before emcc 6.0.0, even if configure tried to make a shared library it would make a fake dynamic library instead and we didn't see this problem. But since then it makes a real dynamic library and then fails to configure.

The fix is if we are configuring pass -Wl,--no-shlib-sigcheck to disable the signature check.

[sbc100]

I think the comment comment above on line 889 is already describing the issue.

Maybe we can just say "For the same reason, we need to disable signature check for shared library symbols."?

Its kind of strange to me that static signature checks would be a warning but not shared signature checks would be an error? I believe the reason static signature checks are only a warning by default is precisely to allow for autoconf/cmake games like this. If that is true then the same logic should apply for shared libraries.

Perhaps if we are going to land this we should add a TOTO with a link to and llvm bug to consider changing the default?

[sbc100]

Thanks for working on this!