Package gtk+-3.0 was not found in the pkg-config search path

Solution 1:

Use search for gtk+-3.0.pc on https://packages.ubuntu.com and then execute

sudo apt-get install libgtk-3-dev

When all packages installed you can try to reinstall pkg-config, and then manually raise pkg-config trigger/hook by

sudo apt-get install --reinstall pkg-config
sudo /usr/share/pkg-config-dpkghook update

and then retry with

pkg-config --cflags gtk+-3.0