`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
