Add AC_POLYFENCE_CIRCLE_INT_SUPPORT_ENABLED gates

[peterbarker]

We've added code which magically converts things stored in this format into floats (#29526) - actually by removing code which kept fences compatible between an old version of ArduPilot and an even older version of ArduPilot.

We have a timeline for removing this integer-circle code:

+// ArduPilot 4.7 no longer stores circle radiuses that look like
+//   integers as integer item types, so any time a fence is saved the
+//   use of the deprecated types is fixed.
+// ArduPilot 4.8 warns if it loads an integer item, warns user to re-upload the fence
+// ArduPilot 4.9 warns if it loads an integer item, warns user to re-upload the fence
+// ArduPilot 4.10 removes support for them

... We've done 4.7, we don't have the warning for 4.8. But at least we can find this code easily after this.

This is a no-compiler-output change.

Setting the define to 0 saves ~150 bytes on CubeOrange