Postgres installed but my C programs won't compile

I had no trouble installing Postgres 6.4 on my RedHat 5.1 Linux machine.
However, the C programs I had previously written to access the SQL
database were not returning the correct values. I tried to make some
changes and re-compile the program but got the following error when I
tried to run the re-compiled program:

[reina ~/c_programs]$ cc process_kinematics.c -o PROCESS_KINEMATICS $SQL
matrix_tools.c gcvspl.o -lm
[reina ~/c_programs]$ PROCESS_KINEMATICS
PROCESS_KINEMATICS: error in loading shared libraries
libpq.so.2.0: cannot open shared object file: No such file or directory

It looks as though SQL can't find my shared libraries for Postgres
anymore. Does anybody know why?

I tried copying my 6.3.2 version back into the proper directories and
the program will compile and re-run correctly.

Anyone know what to do?

Thanks.
-Tony