[patch] Small fixes for warnings and errors

These patches have been sitting around for nearly a year, but I've rebased them onto the trunk and tested them again. They cover several classes of fix:

• minor build improvements
• minor tweaks to feature tests
• enabling stricter compiler warnings, and then fixing those warnings
• fixing mismatched delete/delete[] (bad)
• adding missing virtual destructors (bad)
• removing unused variables
• removing unused variables conditionally when used conditionally
• removing cast warnings with appropriate C++ const/static/reinterpret casts

Most of the fixes are tiny one-liners to fix warnings.
Builds:

• travis
• appveyor