Michal,
I was able to build Stunnel 5.17 on AIX 6.1 using the "build" command, after completely removing the entire section for "-fstack-protector" in "configure" (my "stunnel-5.17.patch" file below). I used the following commands:
mkdir $HOME/stunnel-bin-5.17 gzip -dc stunnel-5.17.tar.gz | tar xvf - patch -p0 <stunnel-5.17.patch cd stunnel-5.17 chmod 755 configure; ./configure --prefix=$HOME/stunnel-bin-5.17 make; make install
However, for 5.18, those same commands (and patch file) no longer result in a successful compile due to AIX 6.1 not accepting the "-u" (Unicode) for pod2man. I looked into 5.17 and it looks like the same Makefiles are used in both (doc/Makefile.in and doc/Makefile.am) so perhaps the top-level Makefile for 5.18 adds the ".pod.8" file to the build process. Removing the "-u" in both files enables the build to complete.
I just tested against Stunnel 5.19 but it looks like the step that ran "pod2html" was skipped by default (i.e., just "make") so I am able to compile stunnel correctly. Thanks for fixing this in Stunnel 5.19! For those that need the .pod.8 man pages on AIX 6.1, you'll need to remove the "-u" in the pod2man command.
Regards, -Rob