The first example in "man pkg-config" is as follows:

Here is a typical usage scenario in a
       Makefile:

       program: program.c
            cc program.c ‘pkg-config --cflags --libs gnomeui‘

The quotes used are not back-quotes.  This is wrong and will certainly lead to confusion.

This is followed by the statement:

       pkg-config retrieves information about packages from  special  metadata
       files. These files are named after the package, with the extension .pc.
       By default, pkg-config looks in the directory prefix/lib/pkgconfig  for
       these  files;  

Note that it says "prefix/lib/pkgconfig", and prefix is underlined.  This should be the actually prefix it was installed to, and should have been replaced during the build process.