Add a CMake option to link to an external IPhreeqc

[SpaceIm]

Currently, source code of IPhreeqc is vendored in PhreeqcRM, and it's hardcoded in the CMakeLists. Would it be possible to add an option to find an external IPhreeqc (with find_package(IPhreeqc REQUIRED)) and link to it (target_link_libraries(PhreeqcRM PRIVATE IPhreeqc::IPhreeqc))?

It's neater for package managers (it avoids ODR violations).