Merge bitcoin#18887: build: enable -Werror=gnu

a30b0a2 build: enable -Werror=gnu (Vasil Dimov)

Pull request description:

  Stop the build if a warning is emitted due to `-Wgnu` and
  `--enable-werror` has been used. As usual - this would help notice such
  a warning that is about to be introduced in new code.

  This is a followup to
  bitcoin#18088 build: ensure we aren't using GNU extensions

ACKs for top commit:
  practicalswift:
    ACK a30b0a2
  Empact:
    ACK bitcoin@a30b0a2

Tree-SHA512: f81b71cf3ee4db88b6f664c571075e0d30800a604f067f44273f256695a1dea533779db2ac859dd0a4cd8b66289c3e45f4aff1cfadfa160a1c354237167b05e2

configure.ac

@@ -372,6 +372,7 @@ if test "x$enable_werror" = "xyes"; then
   if test "x$CXXFLAG_WERROR" = "x"; then
     AC_MSG_ERROR("enable-werror set but -Werror is not usable")
   fi
+  AX_CHECK_COMPILE_FLAG([-Werror=gnu],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=gnu"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=vla],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=vla"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=switch],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=switch"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=thread-safety],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=thread-safety"],,[[$CXXFLAG_WERROR]])