Add missing `extern` declaration for the `module_is_main` flag
Add missing `extern` declaration for the `module_is_main` flag

cython/cython,da-woods/cython,scoder/cython,da-woods/cython,cython/cython,scoder/cython,scoder/cython,da-woods/cython,cython/cython,cython/cython,da-woods/cython,scoder/cython