The man page says: 
"It is typically used to compile and link against one or more libraries.  Here is a typical usage scenario in a Makefile:

       program: program.c
            cc program.c $(pkg-config --cflags --libs gnomeui)"

This makefile code snippet will not work. You either need backquotes or the $(shell...) command to execute pkg-config and substitute its output in its place in the Makefile. I use pkg-config version 0.28