build: add --disable-magic-builddate (reproducible, ccache-stable builds)

New configure option --disable-magic-builddate omits the build date from the
binary.  It is wired via an AC_SUBST'd MAGIC_BUILDDATE_DEFS (not AC_DEFINE), so
it reaches only buildinfo.o rather than @DEFS@ / every command line:

  * enabled (default): buildinfo.o gets -DMAGIC_BUILDDATE="<date>" and defs.mak
    computes the date (SOURCE_DATE_EPOCH-aware);
  * disabled: buildinfo.o gets -DMAGIC_NO_BUILDDATE instead, defs.mak skips the
    date computation entirely (no parse-time `date` spawn), and buildinfo.c
    reports an empty MagicCompileTime.

This gives a fully reproducible build whose one volatile input (the date) is
gone; what remains in buildinfo.o is commit/version/revision, which do not
change second-to-second -- the intended pairing with ccache for day-to-day
development.

Verified both configs: default bakes the date + commit; --disable-magic-builddate
compiles buildinfo.o with -DMAGIC_NO_BUILDDATE (no date), commit still present.

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

scripts/configure

@@ -686,6 +686,7 @@ DOCS_TARGET
 DVIPS
 LATEX
 GCORE
+MAGIC_BUILDDATE_DEFS
 ALLOCA
 EGREP
 GREP
@@ -763,6 +764,7 @@ with_gnu_ld
 enable_assertions
 enable_debug
 enable_compression
+enable_magic_builddate
 with_x
 with_distdir
 with_interpreter
@@ -1426,6 +1428,7 @@ Optional Features:
   --enable-assertions          build with fatal assertions enabled
   --enable-debug               build with fatal debug enabled
   --disable-compression        disable file compression
+  --disable-magic-builddate    omit the build date from the binary (reproducible builds)
   --enable-memdebug            enable memory debugging
   --enable-modular             embed ext2sim and ext2spice packages
   --disable-locking            disable file locking
@@ -6069,6 +6072,20 @@ fi
 
 fi
 
+# Check whether --enable-magic-builddate was given.
+if test "${enable_magic_builddate+set}" = set; then :
+  enableval=$enable_magic_builddate;
+else
+  enable_magic_builddate=yes
+fi
+
+if test "x$enable_magic_builddate" = "xno" ; then
+    MAGIC_BUILDDATE_DEFS="-DMAGIC_NO_BUILDDATE"
+else
+    MAGIC_BUILDDATE_DEFS=""
+fi
+
+
 
 
 

scripts/configure.in

@@ -258,9 +258,24 @@ if test "x$enable_compression" = "xyes" ; then
    AC_CHECK_LIB([z],[gzopen],[
 	AC_DEFINE([HAVE_ZLIB],[1],["zlib compression"])
     	ZLIB_FLAG=" -lz"
-   ])  
+   ])
 fi
 
+dnl Allow the build date to be omitted from the binary (reproducible builds).
+dnl Substituted (not AC_DEFINE'd) so it reaches only buildinfo.o, not @DEFS@ /
+dnl every compile command line.  When disabled, defs.mak also skips computing the
+dnl date entirely (see MAGIC_BUILDDATE_DEFS in scripts/defs.mak.in).
+AC_ARG_ENABLE(magic-builddate,
+[  --disable-magic-builddate    omit the build date from the binary (reproducible builds)],
+[],
+[enable_magic_builddate=yes])
+if test "x$enable_magic_builddate" = "xno" ; then
+    MAGIC_BUILDDATE_DEFS="-DMAGIC_NO_BUILDDATE"
+else
+    MAGIC_BUILDDATE_DEFS=""
+fi
+AC_SUBST(MAGIC_BUILDDATE_DEFS)
+
 dnl Check for some C99 functions  
 
 dnl Built-in round/roundf require CFLAGS -std=c99, but this also

scripts/defs.mak.in

@@ -124,12 +124,19 @@ MAGIC_REVISION	       ?= $(shell cat ${MAGICSRC}/VERSION | cut -d. -f3)
 # 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 ${MAGICSRC} rev-parse HEAD 2>/dev/null)
+# "-DMAGIC_NO_BUILDDATE" when configured with --disable-magic-builddate, else
+# empty.  buildinfo.c reports an empty MagicCompileTime under that define.
+MAGIC_BUILDDATE_DEFS   = @MAGIC_BUILDDATE_DEFS@
+# Only compute the (volatile) build date when it will actually be baked in --
+# skipping it under --disable-magic-builddate avoids the parse-time date spawn.
+ifeq (${MAGIC_BUILDDATE_DEFS},)
 # Honor SOURCE_DATE_EPOCH (reproducible builds -- appimage/npm tarballs) when set,
 # else the current time.  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')
+endif
 
 # This allow inheritence of the values from toplevel Makefile
 export MAGIC_VERSION
@@ -169,7 +176,10 @@ CXXFLAGS               = @CXXFLAGS@
 ifeq (@MAKE_READLINE@,1)
 CPPFLAGS              += -I${MAGICSRC}/readline
 endif
-DFLAGS_MAGICVERSION    = -DMAGIC_VERSION=\"${MAGIC_VERSION}\" -DMAGIC_REVISION=\"${MAGIC_REVISION}\" -DMAGIC_COMMIT=\"${MAGIC_COMMIT}\" "-DMAGIC_BUILDDATE=\"${MAGIC_BUILDDATE}\""
+DFLAGS_MAGICVERSION    = -DMAGIC_VERSION=\"${MAGIC_VERSION}\" -DMAGIC_REVISION=\"${MAGIC_REVISION}\" -DMAGIC_COMMIT=\"${MAGIC_COMMIT}\" ${MAGIC_BUILDDATE_DEFS}
+ifeq (${MAGIC_BUILDDATE_DEFS},)
+DFLAGS_MAGICVERSION   += "-DMAGIC_BUILDDATE=\"${MAGIC_BUILDDATE}\""
+endif
 DFLAGS                 = @extra_defs@ @stub_defs@ @DEFS@ -DGCORE=\"@GCORE@\"
 DFLAGS		      += -DSHDLIB_EXT=\"@SHDLIB_EXT@\" @NDEBUG_defs@ @DEBUG_defs@ ${FEATURE_FLAGS}
 DFLAGS_NOSTUB	       = @extra_defs@             @DEFS@ -DGCORE=\"@GCORE@\"