Fix Libxc 5 detection
Description
Remove flags passed to the linker when detecting Libxc 5 in the corresponding m4 macro, as these are not generic and break the detection of the library in some systems (e.g. MacOS). If a user needs to pass specific flags, this should be done by setting the LIBS_LIBXC environment variable.
News snippet
Fixed Libxc 5 detection in some systems.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.