OK jggimi, thank you.
I already edited
/etc/mk.conf regarding those variables, using [oman=7]ports[/man].
I am using a makefile instead of Makefile now. Simply adding a line
allow me to build a debuggable version.
I used make fake and managed to re-trigger the crash with the thunar from the fake install directory but no .core dumped.
So I make install it and now I have my .core file and the binary to debug it.
I have little (as in "not much but already successfuly used it") experience in gdb. I can see the backtrace.
I guess the next step is to run the debuggable binary in gdb, add breakpoints, read variables then edit sources file, rebuild ,repeat ...
The porter's guide is my next read for sure.
Thank you.
P.S:
Along the way, I was hoping to get .core file from the thunar in the OBSD package. Just for research purposes ...
So I removed the thunar from /usr/local/bin and run
# pkg_check to repair the installation from the packages provided by OBSD. I believe the package used was the one I built myself which defeated the purpose.