different header files installed with meson build
The autotools build installs
/usr/include/libsoup-gnome-2.4/libsoup/soup-cookie-jar-sqlite.h
The meson build doesn't install that but does install
/usr/include/libsoup-gnome-2.4/libsoup/soup-password-manager-gnome.h
/usr/include/libsoup-gnome-2.4/libsoup/soup-proxy-resolver-gnome.h
I guess I'd start by looking here