Fix ambiguous call of constexpr variables

What is the bug

Some compilers (for example, Apple clang, LLVM clang) cannot figure out the constexpr members of a templated class if they are not specialized.

How do you fix it

Use specialized member references instead of this.

How urgent is the bugfix

• I need it as soon as possible
• I can wait for a couple of days
• None of my current code is directly affected

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• Bugfix is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Reviewed by a maintainer
• Incorporate review suggestions
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)
• Prior to merging, please remove any boilerplate from the MR description, retaining only the What is the bug and How do you fix it section to maintain

You are not supposed to merge this request by yourself, the maintainers of libiconmath take care of this action!