support/scripts/dependencies.sh: improve g++ test to avoid misleading output

When g++ is not installed, a misleading error message turns up because
of a bad combination of an unquoted shell variable and control flow.

 ~/buildroot$ make
 You may have to install 'g++' on your build machine
 /home/testuser/buildroot/support/dependencies/dependencies.sh: 136: [: -lt: unexpected operator

[Thomas:
  - fixed commit log, as per the suggestion of Yann E. Morin.
  - don't change existing empty new lines, suggested by Yann.
  - use positive logic in the newly added test, suggested by Yann.]

Signed-off-by: Jens Stimpfle <debian@jstimpfle.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>