strerror_r returns int in emscripten about toml11

[ToruNiina]

I removed strerror that causes this problem because of its high maintenance-cost. So it should be fixed with the latest commit.
I will soon release patch v3.8.1 after CI passes (maybe a few hours later).

---

Background:

strerror is used only when you pass FILE* to toml::parse instead of std::string filename, std::filesystem::path, or std::istream.

There are multiple variants of strerror depending on the environment. The standard version is not thread-safe, so some compilers complain about it (already reported). To avoid this problem, we use environment-specific macros to detect which one is available. But I found that there are multiple environments/conditions where we cannot detect the available variant of strerror using de-facto macro (actually, we have just fixed the same kind of problem yesterday, before the release). There can be more envs where this does not work. The benefit of using strerror is not worth the cost.

I think the FILE* version is less used than other overloads (most of the users may not even know such an overload exists). And the impact is even smaller, since the change does not remove the ability to pass FILE*, but only to return an error number instead of a string if it fails. Still, one can search for the error number, or can get the errno from the exception and call the variant of strerror that is appropriate in the environment.

[ToruNiina]

released.