Fix warnings and enable -Wnon-virtual-dtor warning (#2079)

Fix warnings and enable -Wnon-virtual-dtor warning (#2079)

This commit fixes warnings for an unused variable, comparison of two
different types and add virtual destructors to classes that were
previously missing them. It also enables the -Wnon-virtual-dtor warning
which warns about any class definition with virtual methods that does
not have a virtual destructor (except if the destructor is protected).
This flag is supported by both clang and GCC and not enabled by default.