[CMAKE] Fix missing linker wrap for fcntl64

Summary:
D5881 moved the LFS definition but not the linker wrapping function
associated. This breaks the Linux Gitian build due to symbol check
failure.
This diff moves the linker flag so it applies to libraries as well.
After this diff there is no longer any global or directory level flag
set after including the libraries. A comment is added to help preventing
further issues.
Note that the config is included prior the LFS addition to benefit from
the definition, otherwise it is added blindly.

Depends on D5901.

Test Plan: Run the Linux Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5902