[ng] warns: typos in '_DEPENDENCIES' variables are now reported

[ng] warns: typos in '_DEPENDENCIES' variables are now reported

Detection of those typos had been disabled in commit in commit
'v1.12-295-g9a5f837' of 2012-05-22, "[ng] warns: don't report possible
issues with '_DEPENDENCIES' variables", to avoid spurious errors.

* lib/am/check-typos.am (.am/vartypos/suffixes): Add '_DEPENDENCIES'.
(.am/vartypos/whitelisted-vars): Extend accordingly to avoid spurious
errors.
* t/vartypos.sh: Extend to also test bad usages of '_DEPENDENCIES'
variables.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>