feat: remove strerror from errmsg in FILE* version

parse(FILE*) is a minor overload, but dispatching strerror takes too
much cost. Standard library version is not thread-safe, so some compiler
reports a warning. There are thread-safe versions defined in XSI, GNU,
and Windows. XSI/GNU versions can be detected by macros, but in some
cases, detection-by-macro written in the doc does not work.
Since errno can be obtained from the exception, users can call strerror
that is available in their env if needed. We can just report errno.

toml/exception.hpp

@@ -14,62 +14,14 @@
 namespace toml
 {
 
-namespace detail
-{
-
-inline std::string str_error(int errnum)
-{
-    // C++ standard strerror is not thread-safe.
-    // C11 provides thread-safe version of this function, `strerror_s`, but it
-    // is not available in C++.
-    // To avoid using std::strerror, we need to use platform-specific functions.
-    // If none of the conditions are met, it calls std::strerror as a fallback.
-#ifdef _MSC_VER // MSVC
-    constexpr std::size_t bufsize = 256;
-    std::array<char, bufsize> buf;
-    buf.fill('\0');
-    const auto result = strerror_s(buf.data(), bufsize, errnum);
-    if(result != 0)
-    {
-        return std::string("strerror_s failed");
-    }
-    else
-    {
-        return std::string(buf.data());
-    }
-#elif defined(_GNU_SOURCE) && !(defined(__DARWIN_C_LEVEL) && __DARWIN_C_LEVEL >= 200112L )
-    constexpr std::size_t bufsize = 256;
-    std::array<char, bufsize> buf;
-    buf.fill('\0');
-    const char* result = strerror_r(errnum, buf.data(), bufsize);
-    return std::string(result);
-#elif (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L) || (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE >= 600) || ( defined(__DARWIN_C_LEVEL) && __DARWIN_C_LEVEL >= 200112L ) // macOS 
-    constexpr std::size_t bufsize = 256;
-    std::array<char, bufsize> buf;
-    buf.fill('\0');
-    const int result = strerror_r(errnum, buf.data(), bufsize);
-    if (result != 0)
-    {
-        return std::string("strerror_r failed");
-    }
-    else
-    {
-        return std::string(buf.data());
-    }
-#else // fallback
-    return std::strerror(errnum);
-#endif
-}
-
-} // detail
-
 struct file_io_error : public std::runtime_error
 {
   public:
     file_io_error(int errnum, const std::string& msg, const std::string& fname)
-        : std::runtime_error(msg + " \"" + fname + "\": " + detail::str_error(errnum)),
+        : std::runtime_error(msg + " \"" + fname + "\": errno = " + std::to_string(errnum)),
           errno_(errnum)
     {}
+
     int get_errno() const noexcept {return errno_;}
 
   private: