smyrna Init: squash -Wincompatible-pointer-types

smyrna Init: squash -Wincompatible-pointer-types

OpenGL has an unorthodox API wherein the `gluTessCallback` function’s prototype
indicates it takes a `void(*)(void)`, but its docs¹ explain that the type
actually varies depending on the second argument. As a result, the compiler
(correctly) warns that some of these `gluTessCallback` calls are passing
function pointers that do not have the same ABI. Presumably this works out
because at the end of the day a function pointer is just some bits in C and the
OpenGL implementation branches on the `which` argument and invokes the pointer
correctly. But if OpenGL really wanted to discard type safety this way, it is
not clear to me why they did not make the function pointer argument a `void*`.
Anyway, this commit squashes the compiler warnings which emerge when enabling
this in the CMake build system, failing the build.

Gitlab: related to #1836

¹ This is not the authoritative source, but Microsoft’s docs for their
  implementation provide a good explanation.
  https://docs.microsoft.com/en-us/windows/win32/opengl/glutess