Insert an automatic check for environment variables that override default configurations

[fer-rum]

This is a nicer way to not have to modify the .qmake.config in case you have built livecv somewhere else.
This would concern primarily the variables LIVECV_BIN_PATH and LIVECV_DEV_PATH.

[fer-rum]

I wonder if one could just insert a function for that in the main project's functions.pri