Get the value of environment variable VARIABLE.
VARIABLE should be a string. Value is nil if VARIABLE is undefined in
the environment. Otherwise, value is a string.
If optional parameter FRAME is non-nil, then it should be a
frame. This function will look up VARIABLE in its 'environment
Otherwise, this function searches `process-environment' for
VARIABLE. If it is not found there, then it continues the search
in the environment list of the selected frame.