Makefile: make GBDIR able to be overloaded on the command line
Makefile: make GBDIR able to be overloaded on the command line

Only use the default version if there is not an environment variable
version.

Signed-off-by: Greg Kroah-Hartman <79e2475f81a6317276bf7cbb3958b20d289b78df@kroah.com>

bcousson/gbsim,gregkh/gbsim,gregkh/gbsim,ohporter/gbsim,bcousson/gbsim,projectara/gbsim,projectara/gbsim,bcousson/gbsim