$ yarn add libui-node

$ pkg-config gtk+-3.0 --cflags-only-I | sed s/-I//g

$  sudo apt-get install build-essential libgtk-3-dev

$ echo $PKG_CONFIG_PATH

$ dpkg -L libgtk-3-dev | grep '.pc'
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-wayland-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-unix-print-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-wayland-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-x11-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-x11-3.0.pc

/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-3.0.pc

pkg-config --modversion gtk+3.0

unset PKG_CONFIG_PATH
pkg-config --modversion gtk+-3.0

pkg-config --debug --modversion gtk+-3.0

Option --debug seen
Option --modversion seen
Error printing enabled by default due to use of --version, --libs, --cflags, --libs-only-l, --libs-only-L, --libs-only-other, --cflags-only-I, --cflags-only-other or --list. Value of --silence-errors: 0
Error printing enabled
Adding virtual 'pkg-config' package to list of known packages
Cannot open directory '/usr/local/lib/x86_64-linux-gnu/pkgconfig' in package search path: No such file or directory
Cannot open directory '/usr/local/lib/pkgconfig' in package search path: No such file or directory
Cannot open directory '/usr/local/share/pkgconfig' in package search path: No such file or directory
Scanning directory '/usr/lib/x86_64-linux-gnu/pkgconfig'
[...]

$ pkg-config --debug --modversion gtk+-3.0 2>&1 | egrep "(Cannot open|Scanning) directory"
Cannot open directory '/usr/local/lib/x86_64-linux-gnu/pkgconfig' in package search path: No such file or directory
Cannot open directory '/usr/local/lib/pkgconfig' in package search path: No such file or directory
Cannot open directory '/usr/local/share/pkgconfig' in package search path: No such file or directory
Scanning directory '/usr/lib/x86_64-linux-gnu/pkgconfig'
Scanning directory '/usr/lib/pkgconfig'
Scanning directory '/usr/share/pkgconfig'

export PKG_CONFIG_PATH=/usr/lib/x86_64-linux-gnu/pkgconfig

pkg-config --modversion gtk+-3.0