Using and detecting environment variables as flags

[John Ling]

Hello, I want to be able to read/check the value of an environment 
variable in my Makefile.am.  This would be a variable that I set as part 
of the an [action-if-found] in the AC_SEARCH_LIBS method, that I set in 
configure.ac.

How do I reference such a variable?
In addition, depending on the value of this variable, I then want to 
pass a signal from my Makefile.am to yet another Makefile which is 
called from Makefile.am.  This signal is to tell the second Makefile to 
add an -lz in its LIBS definition.

How do I do this?  In what way can I send such a flag/signal?
Thank You,
John