Fixed installation; other projects can now find module files from iconmath

What is the bug

Any projects dependent on libiconmath were not able to find the associated module files when libiconmath was installed in the system. Additionally, the use of an install_prefix was not supported.

How do you fix it

CMAKE_INSTALL_PREFIX was not used to define INSTALL_INTERFACE. I corrected it now.

How urgent is the bugfix

• I need it as soon as possible
• I can wait for a couple of days
• None of my current codes 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
• 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!