How can I check an environment variable?

[Dr. David Kirkby]

What's the best way to check if an environment variable is set? I want 
to allow a user to set an environment variable

$ SAGE_USE_BROKEN_TOOLS=yes
$ export SAGE_USE_BROKEN_TOOLS

then the configure script check if SAGE_USE_BROKEN_TOOLS is set to 'yes' 
or not and behave differently whether or not it is set.

Obviously the configure script checks things like CC, but can I add a 
check for my own variable?