build: ensure we aren't using GNU extensions

[philmb3487]

• add -Wgnu to compile flags
• rename anonymous structs

In the codebase we are making use of some GNU extensions. We should either remove their usage, or at least amend our CXX compiler checks.

anonymous structs

  ./prevector.h:153:9: warning: anonymous structs are a GNU extension [-Wgnu-anonymous-struct]
          struct {

prevention

To ensure that usage doesn't creep back in we can add -Wgnu to our compile time flags, which will make Clang warn whenever it encounters GNU extensions.

• correct a small issue where there's a class/struct mismatch in CMutableTransaction declarations.

[santos177]

utACK