[ibmibmibm]

Fixes #1621

• MSVC gave C4702 (unreachable code) for the return true; after boost::throw_exception. This stopped builds that treat warnings as errors, for example the Boost.Decimal CI. Keep the result of the NaN search in mode_has_nan in a variable, and return it after the throw, so that no statement follows the throw.
• Without exceptions, Boost requires the program to define boost::throw_exception, so programs that call mode did not link. The Boost.Decimal -fno-exceptions CI job failed for this reason (test_boost_math_univariate_stats does not link with -fno-exceptions decimal#1547), and a Boost.Decimal maintainer asked for the fix in Boost.Math (Define boost::throw_exception in the univariate statistics test decimal#1548). Throw only if BOOST_MATH_NO_EXCEPTIONS is not set, as the policies do, which select errno_on_error by default.
• Without exceptions, the function returns true for data that contain a NaN, and mode gives no result. Standalone mode already did this. Document this.
• Add git_issue_1621.cpp, which runs with and without exceptions, and add it to no_eh_tests.

Tests

• MSVC 19.51 (x64, /W4 /WX): the Boost.Decimal test test_boost_math_univariate_stats fails with C4702 on develop. With this change it compiles and passes, with C++14 and C++latest.
• GCC 16.2 and Clang 23.1, C++14 and C++20, with and without exceptions: the same Boost.Decimal test links and passes, with no change in Boost.Decimal. On develop, the -fno-exceptions build fails with an undefined reference to boost::throw_exception(std::exception const&).
• GCC 16.2: test_univariate_statistics (C++17) and test_univariate_statistics_backwards_compatible (C++14) pass.
• git_issue_1621, GCC 16.2 and Clang 23.1, C++14: passes with exceptions, and with -fno-exceptions -fno-rtti and noeh_support.cpp as in no_eh_tests. On develop, the no-exceptions run aborts. MSVC 19.51 with /EHsc passes.
• GCC 16.2 and Clang 23.1 with -fno-exceptions, in the Boost and the standalone configuration: a program that calls mode links without a definition of boost::throw_exception. mode gives no result for data that contain a NaN, and the correct mode for other data.

[mborland]

Looks good to me. Is there anything else you were planning to add before marking it as ready? Seems sufficient for the issue.