Configure cannot find postgreSQL library

The configure process cannot locate libpq-fe.h, even when it exists on the system:

./configure -qt-sql-psql -fast -v

results in the following warning :

PostgreSQL auto-detection... ()
g++ -c -pipe -g -gdwarf-2 -Wall -W -I../../../mkspecs/macx-g++ -I. -I. -o psql.o psql.cpp
psql.cpp:1:22: error: libpq-fe.h: No such file or directory
psql.cpp: In function 'int main(int, char**)':
psql.cpp:5: error: 'PQescapeBytea' was not declared in this scope
psql.cpp:6: error: 'PQunescapeBytea' was not declared in this scope
make: *** [psql.o] Error 1
PostgreSQL disabled.
PostgreSQL support cannot be enabled due to functionality tests!