build: detect GNU/BSD date at configure time for SOURCE_DATE_EPOCH

The SOURCE_DATE_EPOCH -> date conversion previously relied on a runtime
`date -u -d @EPOCH 2>/dev/null || date -u -r EPOCH` fallback.  That is
unreliable on macOS/*BSD: BSD `date -d` reads -d as a DST flag and silently
prints the wrong time (exit 0), so the `-r` fallback is never reached.

Decide at configure time instead: prefer coreutils `gdate` (GNU date, e.g.
installed on macOS), else a native `date` that really is GNU (verified by the
epoch-0 output containing 1970), else BSD `date -r`.  configure substitutes the
chosen invocation as @SOURCE_DATE_CMD@ and defs.mak appends the epoch.

Verified on Linux (no gdate -> GNU `date -u -d @`): a fixed SOURCE_DATE_EPOCH
yields a byte-identical buildinfo.o across builds; the no-epoch path still
stamps the current local time.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/configure

@@ -693,6 +693,8 @@ GREP
 PERL
 CCACHE_PREFIX_MAP_FLAG
 CCACHE
+SOURCE_DATE_CMD
+GDATE
 SED
 RANLIB
 INSTALL_DATA
@@ -4239,6 +4241,57 @@ $as_echo "$ac_cv_path_SED" >&6; }
   rm -f conftest.sed
 
 
+for ac_prog in gdate
+do
+  # Extract the first word of "$ac_prog", so it can be a program name with args.
+set dummy $ac_prog; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_prog_GDATE+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if test -n "$GDATE"; then
+  ac_cv_prog_GDATE="$GDATE" # Let the user override the test.
+else
+as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_prog_GDATE="$ac_prog"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+fi
+fi
+GDATE=$ac_cv_prog_GDATE
+if test -n "$GDATE"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $GDATE" >&5
+$as_echo "$GDATE" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+  test -n "$GDATE" && break
+done
+
+if test -n "$GDATE"; then
+    SOURCE_DATE_CMD="$GDATE -u -d @"
+elif date -u -d @0 2>/dev/null | grep -q 1970; then
+    SOURCE_DATE_CMD="date -u -d @"
+else
+    SOURCE_DATE_CMD="date -u -r "
+fi
+
+
 # Check whether --enable-ccache was given.
 if test "${enable_ccache+set}" = set; then :
   enableval=$enable_ccache;

scripts/configure.in

@@ -29,6 +29,23 @@ AC_PROG_INSTALL
 AC_PROG_RANLIB
 AC_PROG_SED
 
+dnl SOURCE_DATE_EPOCH -> UTC date, for reproducible builds.  GNU date (Linux, or
+dnl coreutils `gdate` on macOS) converts an epoch with `-d @EPOCH`; BSD date
+dnl (native macOS/*BSD) uses `-r EPOCH`.  Decide here, at configure time, because
+dnl runtime probing is unreliable: BSD `date -d` silently mis-parses `@EPOCH` (it
+dnl reads -d as a DST flag) and prints the wrong time rather than failing.  Prefer
+dnl `gdate`; else a `date` that really is GNU (verified by the epoch-0 output
+dnl containing 1970); else fall back to BSD `date -r`.
+AC_CHECK_PROGS([GDATE], [gdate])
+if test -n "$GDATE"; then
+    SOURCE_DATE_CMD="$GDATE -u -d @"
+elif date -u -d @0 2>/dev/null | grep -q 1970; then
+    SOURCE_DATE_CMD="date -u -d @"
+else
+    SOURCE_DATE_CMD="date -u -r "
+fi
+AC_SUBST([SOURCE_DATE_CMD])
+
 dnl Prefix the compilers with ccache when it is installed (autodetected; opt out
 dnl with --disable-ccache).  Done after the AC_PROG_C* checks above so those
 dnl feature tests run with the plain compiler; only the substituted @CC@/@CXX@

scripts/defs.mak.in

@@ -131,11 +131,13 @@ MAGIC_BUILDDATE_DEFS   = @MAGIC_BUILDDATE_DEFS@
 # skipping it under --disable-magic-builddate avoids the parse-time date spawn.
 ifeq (${MAGIC_BUILDDATE_DEFS},)
 # Honor SOURCE_DATE_EPOCH (reproducible builds -- appimage/npm tarballs) when set,
-# else the current time.  The SOURCE_DATE_EPOCH branch uses UTC so a reproducible
-# stamp does not depend on the builder's zone; the plain-`date` fallback is local.
+# else the current time.  @SOURCE_DATE_CMD@ is the platform's epoch->UTC-date
+# invocation, chosen by configure (GNU `date -u -d @` / `gdate` vs BSD
+# `date -u -r `); the epoch is appended here.  The SOURCE_DATE_EPOCH branch is
+# UTC so a reproducible stamp does not depend on the builder's zone; the plain
+# `date` fallback is local.
 MAGIC_BUILDDATE	       ?= $(shell if [ -n "$$SOURCE_DATE_EPOCH" ]; then \
-			date -u -d "@$$SOURCE_DATE_EPOCH" 2>/dev/null \
-			|| date -u -r "$$SOURCE_DATE_EPOCH" 2>/dev/null; \
+			@SOURCE_DATE_CMD@$$SOURCE_DATE_EPOCH; \
 			else date; fi | tr -d '\r\n')
 endif