GNUInstallDirs: Simplify INFODIR and MANDIR init on BSD platforms

These were using `_GNUInstallDirs_cache_path_fallback` but do not fall
back to `DATAROOTDIR/{info,man}` on BSD platforms.  Convert them to
`_GNUInstallDirs_cache_path`.

Signed-off-by: Cristian Le <git@lecris.dev>