cmake: remove cython 0.29's subinterpreter check

cmake: remove cython 0.29's subinterpreter check

cython 0.29 introduced a check which prevents multiple python
subinterpreters from loading the same module:

https://github.com/cython/cython/commit/7e27c7c

Unfortunately, this completely breaks ceph-mgr.  Until we can
figure out a better long term solution, this commit removes
cython's subinterpreter check, via some careful abuse of the
C preprocessor.

This works because when cython is invoked, it first generates
some C code, then compiles it.  We know it's going to generate
C code including:

  int __Pyx_check_single_interpreter(void) { ... }

and:

  if (__Pyx_check_single_interpreter())
      return NULL;

So, we can do the following:

  #define void0 dead_function(void)
  #define __Pyx_check_single_interpreter(ARG)=ARG ## 0

This replaces the call to __Pyx_check_single_interpreter()
with a literal 0, removing the subinterpreter check.

The void0 dead_function(void) thing is necessary because
the __Pyx_check_single_interpreter() macro also clobbers
that function definition, so we need to make sure it's
replaced with something that works as a function definition.

Fixes: https://tracker.ceph.com/issues/37472
Signed-off-by: Tim Serong <tserong@suse.com>