Makefile: test if dot exists before using it in graph-depends

The 'graph-depends' logic uses the 'dot' program from Graphviz to draw
the dependency graph, but it doesn't check its existence before
starting the generation of the graph, which can lead to user confusion
as reported in:

 http://lists.busybox.net/pipermail/buildroot/2014-June/099278.html



With this commit, we first test if the 'dot' program is available, and
if it's not, we error out with a clear error message:

$ make graph-depends
ERROR: The 'dot' program from Graphviz is needed for graph-depends
make: *** [graph-depends] Error 1

[Peter: send error message to stderr instead]
Reported-by: Dallas Clement <dallas.a.clement@gmail.com>
Cc: Dallas Clement <dallas.a.clement@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>