Merge #18088: build: ensure we aren't using GNU extensions

0ae8f18dfe143051fec6ae10ea7df10142e3ff2f build: add -Wgnu to compile flags (fanquake)
3a0fd7726b8b916de6cce33bb67f48990575f923 Remove use of non-standard zero variadic macros (Ben Woosley)
49f6178c3e5e3ad54a419da9d8523207da17fc64 Drop unused LOG_TIME_MICROS helper (Ben Woosley)
5d4999951ee32e333b511245862628e80f83b703 prevector: Avoid unnamed struct, which is a GNU extension (DesWurstes)

Pull request description:

  Since we [started using](https://github.com/bitcoin/bitcoin/pull/7165) the `ax_cxx_compile_stdcxx.m4` macro we've been passing `[noext]` to indicate that we don't want to use an extended mode, i.e GNU extensions. Speaking to Cory he clarified that the intention was to "require only vanilla c++11 and turn _off_ extension support so they would fail to compile".

  However in the codebase we are currently making use of some GNU extensions. We should either remove there usage, or at least amend our CXX compiler checks. I'd prefer the former.

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

  This is fixed in https://github.com/bitcoin/bitcoin/commit/b849212c1ec01cc8633b8cdcd390da9b1051be0d.

  #### variadic macros

  ```bash
  ./undo.h:57:50: warning: must specify at least one argument for '...' parameter of variadic macro [-Wgnu-zero-variadic-macro-arguments]
              ::Unserialize(s, VARINT(nVersionDummy));
  ```

  This is taken care of in #18087.

  The `LOG_TIME_*` macros introduced in #16805 make use of a [GNU extension](https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html).

  ```bash
  In file included from validation.cpp:22:
  ./logging/timer.h:99:99: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]
      BCLog::Timer<std::chrono::milliseconds> PASTE2(logging_timer, __COUNTER__)(__func__, end_msg, ## __VA_ARGS__)
                                                                                                    ^
  ./logging/timer.h:99:99: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]
  ./logging/timer.h:99:99: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]
  ./logging/timer.h:99:99: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]
  ./logging/timer.h:99:99: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]
  ./logging/timer.h:101:92: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]
      BCLog::Timer<std::chrono::seconds> PASTE2(logging_timer, __COUNTER__)(__func__, end_msg, ## __VA_ARGS__)
                                                                                             ^
  6 warnings generated.
  ```

  This is fixed in 081a0ab64eb442bc85c4d4a4d3bc2c8e97ac2a6d and 612e8e138b97fc5ad2f38847300132a8fc423c3f.

  #### prevention
  To ensure that usage doesn't creep back in we can add [`-Wgnu`](https://clang.llvm.org/docs/DiagnosticsReference.html#wgnu) to our compile time flags, which will make Clang warn whenever it encounters GNU extensions.

  This would close #14130.
  Also related to #17230, where it's suggested we use a GNU extension, the `gnu::pure` attribute.

ACKs for top commit:
  practicalswift:
    ACK 0ae8f18dfe143051fec6ae10ea7df10142e3ff2f -- diff looks correct
  MarcoFalke:
    ACK 0ae8f18dfe143051fec6ae10ea7df10142e3ff2f
  vasild:
    utACK 0ae8f18df
  dongcarl:
    ACK 0ae8f18dfe143051fec6ae10ea7df10142e3ff2f

Tree-SHA512: c517404681ef8edf04c785731d26105bac9f3c9c958605aa24cbe399c649e7c5ee0c4aa8e714fd2b2d335e2fbea4d571e09b0dec36678ef871f0a6683ba6bb7f

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