Allow to override xulrunner location
Allow to override xulrunner location

This change makes it possible to set the XULRUNNER variable before
sourcing environ. This allows to temporarily use a debug version
of xulrunner.

JohnThomson/BloomDesktop,andrew-polk/BloomDesktop,StephenMcConnel/BloomDesktop,andrew-polk/BloomDesktop,gmartin7/myBloomFork,JohnThomson/BloomDesktop,BloomBooks/BloomDesktop,gmartin7/myBloomFork,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop,JohnThomson/BloomDesktop,gmartin7/myBloomFork,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop,andrew-polk/BloomDesktop,gmartin7/myBloomFork,gmartin7/myBloomFork,StephenMcConnel/BloomDesktop,andrew-polk/BloomDesktop,andrew-polk/BloomDesktop,BloomBooks/BloomDesktop,andrew-polk/BloomDesktop,gmartin7/myBloomFork,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop