Cython can now transpile to C++ as an intermediate language

Built-in cython support currently only allows C as an intermediate language, now C++ is also allowed. This can be set via the cython_language option, either on the command line, or in the meson.build files.

project(
  'myproject',
  'cython',
  default_options : ['cython_language=cpp'],
)

or on a per target basis with:

python.extension_module(
  'mod',
  'mod.pyx',
  override_options : ['cython_language=cpp'],
)