gbuild: allow for SHELL to be set by the env variable gb_SHELL
gbuild: allow for SHELL to be set by the env variable gb_SHELL

This allow to build using a non bash shell on system for which
/bin/sh is bash

Change-Id: I276c777d1fca20858219ac95457a367300644b16

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core