FreeBSD: Use /var rather than PREFIX/var

Adriaan de Grootrequested to merge
adridg/cmake:issue-24159 into master

On FreeBSD there should be no /usr/local/var (or other install-prefix var/ subdirectory), there is only /var/ .

Closes: #24159
Topic-rename: GNUInstallDirs-FreeBSD-var

Edited by Brad King

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