Matthias  I cannot follow all you questions as I'm on vacation so briefly:
a) ac_cv_thread is type correct one is with'p'
b) ncurses 
NCURSES_INTERNALS reported again in issue 14598
NCURSES_OPAQUE - i don't have time to explain all details but it could be reproduces on linux with version 2.7 
Even on linux to linux build it is required as it could depend from ncurses library version.

c) setup_info is my proposition to pass detection of readline libs from configure to setup.py . Use of readelf is not portable. Why to test again ? If on host platforms shared library is not in elf format  which tool to use ?

d) importlib.h is build now from executable. Probably we could introduce BUILD_CC . Out of scope for now

e) executable that load packages has to be located in builddir to allow "is python build" to be activated and packages from cross-build environment to be loaded.

After 3 weeks I could answer more.