Revise for future removal of the early rejection checks on HP-UX.
Our code may now work with GCC 4.9 on HP-UX when manually using
`env CXXFLAGS=-D_GLIBCXX_USE_C99` to make the C++11 `std::to_string`
available on this platform.  However, without nightly testing we
cannot officially enable support for the platform.

Issue: #17137
Co-Author: Earle Lowe <elowe@elowe.com>