Fix the atomicity of cy_atomic_int

[ptrrsn]

When building sage, meson.build incorrectly sets all CYSIGNALS_* macros to 0 because of lacks of main function, which leads to a non-atomic declaration of cy_atomic_int.

Moreover, the cy_atomic_int typedefs in struct_signals.h was structured in a way that when it is compiled by a C++ compiler in an environment with a C compiler that supports _Atomic, it would never use std::atomic, which is incorrect and might cause compilation problem when the C++ compiler does not support _Atomic. This problem was hidden by the incorrect configuration of meson.build above but got surfaced after the meson.build fix.

As a context: LLewark/khoca#3

[orlitzky]

int main(void) please, for consistency across the various C standards

[ptrrsn]

Thank you for the review. I changed int main() to int main(void) for the C atomicity checks.