doinstall: Gtk+ binary package not found

In commit 300690d of 2020-01-09, the INSTALL guide was simplified to just reference doinstall for the build.
If I follow the new INSTALL instructions, I get:

$ ./doinstall
Gtk+ binary package not found. Aborting the installation process.

Looking into the doinstall script, I find:

# Checks for the presence of a Gtk+ binary package in gtk-bin/
check_gtk_bin() {
   gtk_bin_dir="`pwd`/gtk-bin"
   if test ! -d "$gtk_bin_dir"; then
      echo "Gtk+ binary package not found. Aborting the installation process."
      exit
   fi
}

I do have the gtk binary package installed on my system (OpenSuSE Tumbleweed):

$ rpm -ql libgtk-3-0-3.24.13+0-3.3.x86_64
/etc/gtk-3.0
/usr/lib64/gtk-3.0
/usr/lib64/gtk-3.0/3.0.0
/usr/lib64/gtk-3.0/3.0.0/immodules
/usr/lib64/gtk-3.0/3.0.0/immodules.cache
/usr/lib64/gtk-3.0/3.0.0/immodules/im-cedilla.so
/usr/lib64/gtk-3.0/3.0.0/immodules/im-cyrillic-translit.so
/usr/lib64/gtk-3.0/3.0.0/immodules/im-ipa.so
/usr/lib64/gtk-3.0/3.0.0/immodules/im-wayland.so
/usr/lib64/gtk-3.0/3.0.0/printbackends
/usr/lib64/gtk-3.0/3.0.0/printbackends/libprintbackend-cloudprint.so
/usr/lib64/gtk-3.0/3.0.0/printbackends/libprintbackend-cups.so
/usr/lib64/gtk-3.0/3.0.0/printbackends/libprintbackend-file.so
/usr/lib64/gtk-3.0/3.0.0/printbackends/libprintbackend-lpr.so
/usr/lib64/gtk-3.0/3.0.0/theming-engines
/usr/lib64/gtk-3.0/modules
/usr/lib64/libgailutil-3.so.0
/usr/lib64/libgailutil-3.so.0.0.0
/usr/lib64/libgdk-3.so.0
/usr/lib64/libgdk-3.so.0.2404.9
/usr/lib64/libgtk-3.so.0
/usr/lib64/libgtk-3.so.0.2404.9
/usr/share/doc/packages/libgtk-3-0
/usr/share/doc/packages/libgtk-3-0/README.SUSE
/usr/share/licenses/libgtk-3-0
/usr/share/licenses/libgtk-3-0/COPYING

I am puzzled: How does this local ./gtk-bin directory come into existence?

Start with INSTALL and the doinstall script, especially check_gtk_bin and its expected gtk-bin directory, then reproduce the reported ./doinstall failure. Trace how gtk-bin is supposed to be supplied and compare that expectation with the documented installation flow. Done means the instructions and doinstall behavior agree about where the GTK binary package comes from.