Fix bug where missing Ford error message was lost

Previously submitted as now declined PR #266. As per the comments on that PR, this is a more robust approach. I’ve done this as a new PR to keep the history clean, rather than have a small commit, then another to effectively undo it, and a merge commit since a different PR has been merged to next since then.

Commit message detail:

This bug is not present in 8.0.3 or 8.0.4_RC but is present in next (1aefb86). When running make -j doc with Ford not installed, the error message reporting that Ford is missing gets lots in the output of the namelist_doc target. This is fixed by having namelist_doc depend explicitly on check_ford_install.

Fixes #98