Do not check for undefined symbols when building the Python modules

Do not check for undefined symbols when building the Python modules

kmod's configure.ac uses the -Wl,--no-undefined linker flag to verify
at link time that all symbols of shared libraries are available, and
that there are no undefined symbols.

This make perfect sense for regular shared libraries. However, for
Python extensions, which will be dlopen()ed inside the Python
interpreter, it makes less sense.

Since Python 3.8, there is a change in python-config script and
Python's pkg-config file: it no longer links Python extensions with
the libpython library. See
https://docs.python.org/dev/whatsnew/3.8.html#debug-build-uses-the-same-abi-as-release-build
which states:

  On the other hand, pkg-config python3.8 --libs no longer contains
  -lpython3.8. C extensions must not be linked to libpython (except on
  Android and Cygwin, whose cases are handled by the script); this
  change is backward incompatible on purpose. (Contributed by Victor
  Stinner in bpo-36721.)

So, when linking the kmod Python extensions, it currently fails with
numerous unresolved symbols, that were previously provided by
libpython:

/home/test/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/powerpc64-buildroot-linux-gnu/7.4.0/../../../../powerpc64-buildroot-linux-gnu/bin/ld: libkmod/python/kmod/.libs/list_la-list.o: in function `__Pyx_PyObject_GetAttrStr':
list.c:(.text.__Pyx_PyObject_GetAttrStr+0x48): undefined reference to `PyObject_GetAttr'
/home/test/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/powerpc64-buildroot-linux-gnu/7.4.0/../../../../powerpc64-buildroot-linux-gnu/bin/ld: libkmod/python/kmod/.libs/list_la-list.o: in function `__pyx_tp_dealloc_4kmod_4list_ModListItem':
list.c:(.text.__pyx_tp_dealloc_4kmod_4list_ModListItem+0x78): undefined reference to `PyObject_CallFinalizerFromDealloc'
/home/test/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/powerpc64-buildroot-linux-gnu/7.4.0/../../../../powerpc64-buildroot-linux-gnu/bin/ld: libkmod/python/kmod/.libs/list_la-list.o: in function `__pyx_tp_dealloc_4kmod_4list_ModList':
list.c:(.text.__pyx_tp_dealloc_4kmod_4list_ModList+0x30): undefined reference to `PyErr_Fetch'

[Complete log at http://autobuild.buildroot.net/results/79a/79a5a0398723e8cfea0d0aa3dec5f7649aee4c63/build-end.log]

Linking with libpython is no longer recommended: those symbols should
remain unresolved in the Python extensions, as they wil be properly
resolved when the Python extension gets loaded into the Python
interpreter.

Since we want to keep -Wl,--no-undefined globally in kmod, we leave
the configure.ac file unchanged, and instead, specifically in the
LDFLAGS used to build the Python extensions, we override
-Wl,--no-undefined with -Wl,-z,undefs. Ideally, -Wl,--no-undefined is
the same as -Wl,-z,defs, and the effect of these options can be
canceled on the linker command line by a following -Wl,-z,undefs (see
the ld man page for details).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Victor Stinner <victor.stinner@gmail.com>