Test for environment variable

Makefile

#Taken from 
#http://stackoverflow.com/questions/4728810/makefile-variable-as-prerequisite
#Remember that each make target is run within a separate sub-shell, which defaults to sh (not bash)
#so bash-specific default environment variables like $HOSTNAME will *not* be accessible *unless* 
# they are exported *before* `make' is run; ie, `export HOSTNAME=$HOSTNAME'

change-hostname: assertenvar-USER assertenvar-TESTVAR assertenvar-HOSTNAME assertenvar-TEF              
                                                                                
assertenvar-%:                                                                        
    @if [ -z ${${*}} ] ; then \                                                 
        echo "Environment variable $* not set"; \                               
            exit 1; \                                                           
    fi