just type in 'pg_config' ,without quotes in terminal and see if it runs. if
not, you gotta find it. For instance by using:
find /usr -name pg_config
than if it does come up with whereabouts of it - stick it into PG_CONFIG env
variable:
export PG_CONFIG=/path/path/pg_config
and rerun configure in the same terminal (important, the env variable
PG_CONFIG will only be seen in that scope/terminal).