Make ConflictSet non-copyable in C++98/C++03

`ConflictSet(const ConflictSet&)` and `operator=(const ConflictSet&)` were
only deleted for C++11 and later. In C++98/C++03 the compiler implicitly
generated public copy operations, so copying a ConflictSet shared the opaque
`Impl*` and caused a double-free on destruction.

Declare both operations private and leave them undefined when
`__cplusplus <= 199711L`, matching the standard pre-C++11 idiom for
move-only types. Guard the declarations with `defined(__cplusplus)` so
they are not exposed to C90 compilation units.

Closes #48

include/ConflictSet.h

@@ -132,6 +132,13 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
 
 private:
   Impl *impl;
+#if defined(__cplusplus) && __cplusplus <= 199711L
+  /* Declared private and left undefined to prevent copying in C++98/C++03.
+     The compiler would otherwise implicitly generate public copy operations,
+     which share the opaque Impl* and cause a double-free. */
+  ConflictSet(const ConflictSet &);
+  ConflictSet &operator=(const ConflictSet &);
+#endif
 };
 } /* namespace weaselab */