I remember old emails where, with your help, I had managed to compile
without thirdparty. I can't find that thread anymore; does anyone
remember what it was? The fact is that I forgot how to do such a
build.

you can look into "bld" directory - BSDs build work exactly without thirdparty (shortened):

./configure --with-single-user \
--disable-static-build --without-thirdparty --without-libdpx

May be we should add this to manual .

I have this staged to add to the Manual GIT. It will look like this (but it will mostly be missed in reading anyway):

1.3.4 Notes about Building from Git in your Customized Environment

Getting a build to work in a custom environment is not easy. If you have already installed libraries which are normally in the thirdparty build, getting them to be recognized means you have to install the devel version so the header ﬁles which match the library interfaces exist. If you want to build using only the thirdparty libraries installed in your system, just include "–without-thirdparty" to your conﬁgure script. For example:

./confgure --with-single-user --disable-static-build --without-thirdparty

Below is the list of thirdparty builds, but this list may have changed over ...