build: honor SOURCE_DATE_EPOCH for the build date; graceful git fallback

MAGIC_BUILDDATE now derives from SOURCE_DATE_EPOCH (UTC) when it is set,
falling back to the current local time otherwise.  This makes the date baked
into buildinfo.o reproducible for the appimage/npm tarballs (cf. npm/pack.sh,
which already normalizes mtimes).

Also add `2>/dev/null` to the MAGIC_COMMIT git call so a source tarball or a
non-work-tree ${MAGICSRC} yields an empty commit rather than a git error.

Only buildinfo.o consumes these (see the prior commit), so the shell still
runs at defs.mak parse time but affects just that one unit's define.

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

scripts/defs.mak.in

@@ -121,8 +121,15 @@ TCL_LIB_DIR	       = @TCL_LIB_DIR@
 
 MAGIC_VERSION	       ?= $(shell cat ${MAGICSRC}/VERSION | cut -d. -f1-2)
 MAGIC_REVISION	       ?= $(shell cat ${MAGICSRC}/VERSION | cut -d. -f3)
-MAGIC_COMMIT	       ?= $(shell git -C ${MAGICSRC} rev-parse HEAD)
-MAGIC_BUILDDATE	       ?= $(shell date | tr -d '\r\n')
+# git may be absent (source tarball) or ${MAGICSRC} not a work-tree: fall back to
+# empty rather than emitting an error.
+MAGIC_COMMIT	       ?= $(shell git -C ${MAGICSRC} rev-parse HEAD 2>/dev/null)
+# Honor SOURCE_DATE_EPOCH (reproducible builds -- appimage/npm tarballs) when set,
+# else the current time.  UTC so the stamp does not depend on the builder's zone.
+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; \
+			else date; fi | tr -d '\r\n')
 
 # This allow inheritence of the values from toplevel Makefile
 export MAGIC_VERSION