support/dependencies: avoid spurious warning on print-vars

When calling 'printvars', the 'suitable-host-package' macro is printed
(a macro is just a variable like the others, after all, just with some
parameters). Because it is printed as a variable, it is missing its
parameters, but it still tries to evaluate the $(shell) construct.

This causes spurious warning:

    make[1]: support/dependencies/check-host-.sh: Command not found

Only try and call the script if there is actually a tool to check for.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>