math/nlopt: Fix build with Octave installed

cmake requires at least C++17 to build correctly if math/octave is
already installed.

math/octave is now a fixed dependency for nlopt, as this is the
only way to guarantee that Octave packages such as
math/octave-forge-mboct-fem-pkg will find a sufficiently
complete nlopt.

PR: 289110
Reported by: stephen@
MFH: 2025Q3