Several tests specify use of C++98 mode since commit b0f277db (HP aCC:
record compiler flag for Ansi C++98 support for version 3.80 onwards,
2014-04-15, v3.1.0-rc1~550^2).  The motivating use case for those
changes was dropped by commit 49640d36 (HP-UX: Drop support for
building CMake on HP-UX, 2017-08-04, v3.10.0-rc1~304^2), so remove the
use of C++98 mode too.  Also remove code that, on some compilers, used
C++11 instead of C++98, because we can now just use their default mode.