[build-system] Fix gfortran module search for libxc

Informally reported in #89 (closed).

Description

Octopus fails to build when using packaged libxc because the *.mod files in RHEL based distros (not sure about Debian ones) are located in /usr/lib64/gfortran/modules/. But this is not in search path of gfortran.

Note

This is a temporary fix. Permanent fixes should be either:

• The gfortran search path should be updated on the fedora project repo and equivalently if other distros have this issue
• Eventually switch to cmake, where we can import the configuration native of the OS (find_package) or using pkg-config (FindPkgConfig).

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.