This required some libraries. I installed them all into _usr_local/stow, and used stow to create the links to _usr_local. This means that the pkg-config files ended up in _usr_local/lib/pkgconfig. For the various configure scripts to run, I had to tell it about the pkgconfig directory was where all the .pc files were:
export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig
Had to get libsigc++ 2.0.6, first.
./configure make make install prefix=/usr/local/stow/libsigc++ su cd /usr/local/stow stow libsigc++ ldconfig exit
Not sure whether the ldconfig is needed. I just do it when I install new libraries.
Had to get gtkmm, which depends on glibmm-2.4. First I tried glibmm-2.5, but it turns out that 2.5 depends on glib-2.0 2.6.0, but version 2.4.2 is installed on Slackware. *ARGH!* So I tried glibmm-2.4.5 instead. And that configured correctly.
./configure make make install prefix=/usr/local/stow/glibmm su cd /usr/local/stow stow glibmm ldconfig exit
Not for gtkmm 2.4.8.
./configure make make install prefix=/usr/local/stow/gtkmm su cd /usr/local/stow stow gtkmm ldconfig exit
Got Inkscape 0.40, next.
./configure make make install prefix=/usr/local/stow/inkscape su cd /usr/local/stow stow inkscape ldconfig exit
Done. 😄