droption.h build errors with some clang versions

Seeing this with an internal toolchain based on a recent clang. I plan to fix this now; filing to document:

ext/droption/droption.h:140:7: error: 'droption_parser_t' has virtual functions but non-virtual destructor [-Werror,-Wnon-virtual-dtor]
class droption_parser_t {
      ^ 

ext/droption/droption.h:403:29: error: 'droption_t<bool>' has virtual functions but non-virtual destructor [-Werror,-Wnon-virtual-dtor] 
template <typename T> class droption_t : public droption_parser_t {