Fix FindLibxc.cmake to use /usr/include as include directory

[yizeyi18]

pkg-config would ignore flags like -I/usr/include in cflags; as ${prefix}_INCLUDE_DIRS is initialized from pkg-config --cflags-only-I ${pkgname}, it would be empty as only -I/usr/include in cflags. However, under this situation, cmake would still mark ${prefix}_FOUND as true, and without these global-visible includes no compilation issues arise; thus, this commit turns off detect of Libxc_INCLUDE_DIRS, and use Libxc_found instead.

Reminder

• Have you linked an issue with this pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

Fix #3761

What's changed?

• Changed filter condition of find_package_handle_standard_args in cmake/FindLibxc.cmake.

Any changes of core modules? (ignore if not applicable)

No.