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>
(cherry picked from commit 77304e5143bf77e9fd2f8125807bbf9665cda8bc)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/dependencies/dependencies.mk

@@ -15,7 +15,7 @@ else
 # script should use 'which' to find a candidate. The script should return
 # the path to the suitable host tool, or nothing if no suitable tool was found.
 define suitable-host-package
-$(shell support/dependencies/check-host-$(1).sh $(2))
+$(if $(1),$(shell support/dependencies/check-host-$(1).sh $(2)))
 endef
 endif
 # host utilities needs host-tar to extract the source code tarballs, so