Add option for external libs in CMakeLists

[jeanwsr]

Recently libxc_itrf2 is added, which links to libxc. But in some cases one may not place the libxc in pyscf/lib/deps. So an option for libxc directory should be added.

[MatthewRHermes]

Is this necessary? If LibXC were to be installed externally, wouldn't it naturally already be in LD_LIBRARY_PATH? I don't see this kind of option in the PySCF core module.

[jeanwsr]

Yes, it is in the LD_LIBRARY_PATH. But that's not sufficient to make it be recognized by cmake. cmake uses the link_directories() command to determine the path of dynamical lib to be used in "cc -L xxx -l xxx". I'm not sure whether there's any other way to do it, like any cmake options?

In pyscf core program, the CMakeLists add CMAKE_INSTALL_PREFIX/lib to link_directories() (See https://github.com/pyscf/pyscf/blob/a0665c4a7bf54e33f01295b3eea390be7a17d76d/pyscf/lib/CMakeLists.txt#L121), which allows users using -DCMAKE_INSTALL_PREFIX to specify the libxc path. This is documented in https://pyscf.org/user/install.html#install-without-internet . The option name is a little bit weird because pyscf itself does not use CMAKE_INSTALL_PREFIX to install anything. Probably it comes from the CMAKE_INSTALL_PREFIX of libcint and libxc in the same installing instruction.

In summary, I think it's necessary to provide the same cmake option as pyscf core do. The name of the option can be discussed: to use XC_DIR as it's simple, or use CMAKE_INSTALL_PREFIX to keep consistency with pyscf, or something else like EXTERNAL_DEPS_DIR.

[MatthewRHermes]

I see. I would suggest copying the PySCF core-module solution to this issue if possible, just to minimize confusion.

[jeanwsr]

Copied.

[MatthewRHermes]

Cool, thanks