don't always force build of givaro+fflas_ffpack if linbox must be built #37348
Description
In #36997 the change
db7703c47b27 (Matthias Koeppe 2024-01-02 18:54:05 -0800 11) ], [dnl REQUIRED_CHECK
db7703c47b27 (Matthias Koeppe 2024-01-02 18:54:05 -0800 12) ], [dnl PRE
db7703c47b27 (Matthias Koeppe 2024-01-02 18:54:05 -0800 13) ], [dnl POST
db7703c47b27 (Matthias Koeppe 2024-01-02 18:54:05 -0800 14) sage_spkg_install_fflas_ffpack=$sage_spkg_install_linbox
db7703c47b27 (Matthias Koeppe 2024-01-02 18:54:05 -0800 15) sage_spkg_install_givaro=$sage_spkg_install_linbox
has broken the case where givaro and/or fflas_ffpack from the system are OK, but linbox is not.
If the "old" set of givaro+fflas_ffpack is OK, but linbox is not, then only linbox ought to be built, not the whole trio.
This might not the case for the "new" set, as our linbox is probably too old to use "new" givaro+fflas_ffpack.