First find where gtk+-2.0.pc is with either:
find / -name gtk+-2.0.pc 2> /dev/null
If neither give you a path than you need to install gtk+. If gtk+ is installed then continue on.
Depending on where you installed gtk the .pc file will most likely be in /usr/lib/pkgconfig or /lib/pkgconfig. For example, if gtk+-2.0.pc was found in /usr/lib/pkgconfig export this path to the PKG_CONFIG_PATH variable:
Finally, either try running through the install again and cross your fingers or double check that you set it correctly first with:
pkg-config --modversion gtk+-2.0
Which should output the version number.