I don’t run AIX, but my understanding is there are three distinct branches (2.7, 3.5, and 3.6+). Some of the following is guessed, so please correct me if I am wrong:

Python 2.7:
_sysconfigdata.py incorrectly created with LDSHARED = Modules/ld_so_aix.
Result: Installed distutils fails (unless you’re in the build tree).
Fix: Apply David’s issue18235.patch + Michael H’s msg219888 fix.

Python 3.5:
David’s issue18235.patch already applied.
_sysconfigdata.py incorrectly created with BLDSHARED = $(LIBPL)/ld_so_aix, i.e. the installed location.
Distutils does not refer to _sysconfigdata.py.
Result: Distutils unaffected, but running from the build tree, sysconfig.get_config_var("LDSHARED") would incorrectly use the installed location.
Fix: Apply Michael H’s msg219888.

Python 3.6+:
Distutils does use _sysconfigdata*.py, thanks to revision 3fa8aebed636.
Result: Distutils shouldn’t work unless Python is already installed. Since the 3.6 and 3.x buildbots pass test_distutils, does that mean that both those versions of Python are already installed?
Fix: Should also be fixed by merging msg219888 from 3.5.

Marc-Andre, regarding configure.ac and absolute vs relative paths, I suggest you open a separate bug. That is an orthogonal issue, and this bug is already too complicated. But also keep in mind Issue 18235. You want the build tree, not the source tree.

Also, if you mean to drop the assignment in sysconfig, I think that will break in-tree usage of sysconfig (3.5+) and distutils (3.6+), unless Python happens to also be installed.