bpo-44133: Link Python executable with object files (GH-30556)

When Python is built without --enable-shared, the "python" program is
now linked to object files, rather than being linked to the Python
library (libpython.a), to make sure that all symbols are exported.
Previously, the linker omitted some symbols like the Py_FrozenMain()
function.

When Python is configured with --without-static-libpython, the Python
static library (libpython.a) is no longer built.

* Check --without-static-libpython earlier in configure.ac
* Add LINK_PYTHON_OBJS and LINK_PYTHON_DEPS variables to Makefile.
* test_capi now ensures that the "Py_FrozenMain" symbol is exported.