Merge #17880: build: add -Wdate-time to Werror flags

b0a2540 build: add Wdate-time to Werror flags (fanquake)

Pull request description:

  `-Wdate-time`
  Warn when macros __TIME__, __DATE__ or __TIMESTAMP__ are encountered as
  they might prevent bit-wise-identical reproducible compilations.

  This is supported by [GCC](https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html) and [Clang](https://clang.llvm.org/docs/DiagnosticsReference.html#wdate-time).

  Example output:
  ```bash
    CXX      bitcoind-bitcoind.o
  bitcoind.cpp:48:20: warning: expansion of date or time macro is not reproducible [-Wdate-time]
      printf("%s\n", __TIMESTAMP__);
                     ^
  bitcoind.cpp:49:20: warning: expansion of date or time macro is not reproducible [-Wdate-time]
      printf("%s\n", __TIME__);
                     ^
  bitcoind.cpp:50:20: warning: expansion of date or time macro is not reproducible [-Wdate-time]
      printf("%s\n", __DATE__);
                     ^
  3 warnings generated.
  ```

ACKs for top commit:
  practicalswift:
    ACK b0a2540 -- diff looks correct and guarding against potential non-reproducibility is good :)
  promag:
    Tested ACK b0a2540 on macos with clang. Already had `--enable-werror`, added a wild `printf("%s\n", __TIMESTAMP__)` and got the following error:
  laanwj:
    ACK b0a2540
  hebasto:
    ACK b0a2540

Tree-SHA512: b3a0b426e06dcd0c0baa94118c31158760b9690a8d0a15b5a2d544cb0879522e02817e134ef7346c707de09719818fc7e4bad1b3ad6b2dfe5e3c4169cdf5cb0d

Author: laanwj

configure.ac

@@ -329,6 +329,7 @@ if test "x$enable_werror" = "xyes"; then
   AX_CHECK_COMPILE_FLAG([-Werror=switch],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=switch"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=thread-safety-analysis],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=thread-safety-analysis"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=unused-variable],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=unused-variable"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Werror=date-time],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=date-time"],,[[$CXXFLAG_WERROR]])
 fi
 
 if test "x$CXXFLAGS_overridden" = "xno"; then
@@ -342,6 +343,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wrange-loop-analysis],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wrange-loop-analysis"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wredundant-decls],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wredundant-decls"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wunused-variable],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunused-variable"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wdate-time],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdate-time"],,[[$CXXFLAG_WERROR]])
 
   dnl Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
   dnl unknown options if any other warning is produced. Test the -Wfoo case, and