Confusing build errors on linux (suggested change to readme)

[ForestJohnson]

$ go build
# pkg-config --cflags  -- cairo cairo-gobject gobject-2.0
Package cairo was not found in the pkg-config search path.
Perhaps you should add the directory containing `cairo.pc'
to the PKG_CONFIG_PATH environment variable
No package 'cairo' found
Package cairo-gobject was not found in the pkg-config search path.
Perhaps you should add the directory containing `cairo-gobject.pc'
to the PKG_CONFIG_PATH environment variable
No package 'cairo-gobject' found
pkg-config: exit status 1
# pkg-config --cflags  -- gdk-3.0 glib-2.0 gobject-2.0
Package gdk-3.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `gdk-3.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'gdk-3.0' found
pkg-config: exit status 1

I suggest a change something like this to the readme although im not 100% sure what should go there, at least this worked for me on ubuntu:

https://github.com/ForestJohnson/dialog/pull/1/files

[sqweek]

These were indirect dependencies via gotk3. I've just stopped using that package in favour of direct cgo and updated the readme to note the dependency on gtk3 headers/libraries.