FLTK loads the GTK filechooser dynamically if it is found on the system
and if it is not disabled by the user or system option
Fl::option(OPTION_PRINTER_USES_GTK). That's why it doesn't show up in
linked dependencies. Look for 'libgtk' in the FLTK sources, e.g. `git
grep libgtk`. Looks like the library is either "libgtk-3" or
"libgtk-x11-2.0" but this may not help you.

$ lddtree /usr/lib/x86_64-linux-gnu/libgtk-3.so
/usr/lib/x86_64-linux-gnu/libgtk-3.so (interpreter => None)
    libgdk-3.so.0 => /lib/x86_64-linux-gnu/libgdk-3.so.0
        libxkbcommon.so.0 => /lib/x86_64-linux-gnu/libxkbcommon.so.0
        libwayland-client.so.0 => /lib/x86_64-linux-gnu/libwayland-client.so.0
            libffi.so.8 => /lib/x86_64-linux-gnu/libffi.so.8
            libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0
        libwayland-cursor.so.0 => /lib/x86_64-linux-gnu/libwayland-cursor.so.0
        libwayland-egl.so.1 => /lib/x86_64-linux-gnu/libwayland-egl.so.1
        libXext.so.6 => /lib/x86_64-linux-gnu/libXext.so.6
        libXcursor.so.1 => /lib/x86_64-linux-gnu/libXcursor.so.1
            libXrender.so.1 => /lib/x86_64-linux-gnu/libXrender.so.1
        libXdamage.so.1 => /lib/x86_64-linux-gnu/libXdamage.so.1
        libXcomposite.so.1 => /lib/x86_64-linux-gnu/libXcomposite.so.1
        libXrandr.so.2 => /lib/x86_64-linux-gnu/libXrandr.so.2
        libXinerama.so.1 => /lib/x86_64-linux-gnu/libXinerama.so.1
    libgmodule-2.0.so.0 => /lib/x86_64-linux-gnu/libgmodule-2.0.so.0
    libglib-2.0.so.0 => /lib/x86_64-linux-gnu/libglib-2.0.so.0
        libpcre2-8.so.0 => /lib/x86_64-linux-gnu/libpcre2-8.so.0
    libgobject-2.0.so.0 => /lib/x86_64-linux-gnu/libgobject-2.0.so.0
    libgio-2.0.so.0 => /lib/x86_64-linux-gnu/libgio-2.0.so.0
        libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1
        libmount.so.1 => /lib/x86_64-linux-gnu/libmount.so.1
            libblkid.so.1 => /lib/x86_64-linux-gnu/libblkid.so.1
            ld-linux-x86-64.so.2 => /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
        libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1
    libpangocairo-1.0.so.0 => /lib/x86_64-linux-gnu/libpangocairo-1.0.so.0
    libpango-1.0.so.0 => /lib/x86_64-linux-gnu/libpango-1.0.so.0
        libthai.so.0 => /lib/x86_64-linux-gnu/libthai.so.0
            libdatrie.so.1 => /lib/x86_64-linux-gnu/libdatrie.so.1
    libharfbuzz.so.0 => /lib/x86_64-linux-gnu/libharfbuzz.so.0
        libfreetype.so.6 => /lib/x86_64-linux-gnu/libfreetype.so.6
            libpng16.so.16 => /lib/x86_64-linux-gnu/libpng16.so.16
            libbrotlidec.so.1 => /lib/x86_64-linux-gnu/libbrotlidec.so.1
                libbrotlicommon.so.1 => /lib/x86_64-linux-gnu/libbrotlicommon.so.1
        libgraphite2.so.3 => /lib/x86_64-linux-gnu/libgraphite2.so.3
    libcairo.so.2 => /lib/x86_64-linux-gnu/libcairo.so.2
        libpixman-1.so.0 => /lib/x86_64-linux-gnu/libpixman-1.so.0
        libxcb-shm.so.0 => /lib/x86_64-linux-gnu/libxcb-shm.so.0
        libxcb.so.1 => /lib/x86_64-linux-gnu/libxcb.so.1
            libXau.so.6 => /lib/x86_64-linux-gnu/libXau.so.6
            libXdmcp.so.6 => /lib/x86_64-linux-gnu/libXdmcp.so.6
                libbsd.so.0 => /lib/x86_64-linux-gnu/libbsd.so.0
                    libmd.so.0 => /lib/x86_64-linux-gnu/libmd.so.0
        libxcb-render.so.0 => /lib/x86_64-linux-gnu/libxcb-render.so.0
    libpangoft2-1.0.so.0 => /lib/x86_64-linux-gnu/libpangoft2-1.0.so.0
    libfontconfig.so.1 => /lib/x86_64-linux-gnu/libfontconfig.so.1
        libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1
    libfribidi.so.0 => /lib/x86_64-linux-gnu/libfribidi.so.0
    libcairo-gobject.so.2 => /lib/x86_64-linux-gnu/libcairo-gobject.so.2
    libgdk_pixbuf-2.0.so.0 => /lib/x86_64-linux-gnu/libgdk_pixbuf-2.0.so.0
        libjpeg.so.62 => /lib/x86_64-linux-gnu/libjpeg.so.62
    libatk-1.0.so.0 => /lib/x86_64-linux-gnu/libatk-1.0.so.0
    libepoxy.so.0 => /lib/x86_64-linux-gnu/libepoxy.so.0
        libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6
    libXi.so.6 => /lib/x86_64-linux-gnu/libXi.so.6
    libX11.so.6 => /lib/x86_64-linux-gnu/libX11.so.6
    libatk-bridge-2.0.so.0 => /lib/x86_64-linux-gnu/libatk-bridge-2.0.so.0
        libatspi.so.0 => /lib/x86_64-linux-gnu/libatspi.so.0
        libdbus-1.so.3 => /lib/x86_64-linux-gnu/libdbus-1.so.3
            libsystemd.so.0 => /lib/x86_64-linux-gnu/libsystemd.so.0
                libcap.so.2 => /lib/x86_64-linux-gnu/libcap.so.2
                libgcrypt.so.20 => /lib/x86_64-linux-gnu/libgcrypt.so.20
                    libgpg-error.so.0 => /lib/x86_64-linux-gnu/libgpg-error.so.0
                liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5
                libzstd.so.1 => /lib/x86_64-linux-gnu/libzstd.so.1
                liblz4.so.1 => /lib/x86_64-linux-gnu/liblz4.so.1
    libXfixes.so.3 => /lib/x86_64-linux-gnu/libXfixes.so.3
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6