manual generation: check dependencies first

To generate the manual, you need a few tools. If these are not present,
pretty cryptic error messages are given.
This patch adds a simple check for these dependencies, before attempting to
build the manual.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>