build: isolate build-info defines into utils/buildinfo.c (ccache-friendly)

MAGIC_VERSION / MAGIC_REVISION / MAGIC_COMMIT / MAGIC_BUILDDATE were on the
global DFLAGS, so every object's compile command line carried them.  Because
the command line is part of ccache/sccache's hash key and MAGIC_BUILDDATE
changes every second, that meant a cache miss on every unit on every build;
it also smeared the baked-in date/commit across "whenever each file last
recompiled".

Move the four values into a single translation unit:
  * utils/magic_buildinfo.h -- extern MagicVersion/MagicRevision/MagicCommit/
    MagicCompileTime (MagicCommit is new; the other three moved here from
    utils/magic.h, which now includes this header).
  * utils/buildinfo.c -- the ONLY unit compiled with the version defines
    (target-specific `buildinfo.o: DFLAGS += ${DFLAGS_MAGICVERSION}` in
    utils/Makefile.in); it defines the four globals.  ${DFLAGS_MAGICVERSION}
    is removed from the global DFLAGS/DFLAGS_NOSTUB.

Consumers updated to read the runtime symbol instead of the compile-time
macro:
  * magicTop.c / tclmagic.c no longer define the globals (the MAGIC_WRAPPER
    duplicate-symbol guard is gone -- buildinfo.o owns them for every variant,
    including WASM, which links both mains).
  * Tcl_PkgProvide/PkgRequire in router/ext2spice/lef/ext2sim/plot/tclmagic
    now pass MagicVersion.
  * extflat: EFVersion (a static-initialized copy of MAGIC_VERSION) is dropped;
    EFread.c compares the .ext version against MagicVersion directly -- the same
    value ExtCell.c already *writes* into .ext files, so read and write are now
    consistent.

Verified: only buildinfo.o carries -DMAGIC_* (hash/DBio/windCmdSZ carry none);
Tcl (375 files) and --without-tcl builds rc=0, version/commit/date embedded in
tclmagic.so, magicTop.o no longer defines MagicVersion; in-tree source clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

utils/Makefile.in

@@ -9,7 +9,7 @@ srcdir    = @srcdir@
 VPATH     = @srcdir@
 MAGICDIR  = @top_builddir@
 LIB_SRCS  = LIBdbio.c LIBmain.c LIBtextio.c
-SRCS      = args.c child.c dqueue.c finddisp.c flock.c flsbuf.c fraction.c \
+SRCS      = args.c buildinfo.c child.c dqueue.c finddisp.c flock.c flsbuf.c fraction.c \
             geometry.c getrect.c hash.c heap.c ihash.c list.c lookup.c \
 	    lookupany.c lookupfull.c macros.c main.c malloc.c match.c \
 	    maxrect.c netlist.c niceabort.c parser.c path.c pathvisit.c \
@@ -23,3 +23,9 @@ DFLAGS += ${GR_DFLAGS}
 CFLAGS += ${GR_CFLAGS}
 
 include ${MAGICSRC}/rules.mak
+
+# buildinfo.c is the ONLY unit that consumes the version defines; they are no
+# longer on the global DFLAGS (that kept the ever-changing MAGIC_BUILDDATE off
+# every other compile command line, so ccache/sccache keys stay stable).  Apply
+# them just here, target-specifically.
+buildinfo.o: DFLAGS += ${DFLAGS_MAGICVERSION}

utils/buildinfo.c

@@ -0,0 +1,31 @@
+/*
+ * buildinfo.c --
+ *
+ * The single translation unit that captures the build-information defines.
+ *
+ * This is the ONLY file compiled with -DMAGIC_VERSION / -DMAGIC_REVISION /
+ * -DMAGIC_COMMIT / -DMAGIC_BUILDDATE (see the buildinfo.o recipe in
+ * utils/Makefile.in).  Isolating them here keeps those defines -- especially the
+ * ever-changing MAGIC_BUILDDATE -- off every other compile command line, so
+ * ccache/sccache keys stay stable and the version/date/commit baked into the
+ * binary are consistent rather than smeared across whenever each object last
+ * recompiled.
+ *
+ * Consumers include "utils/magic_buildinfo.h" and read these externs.
+ */
+
+#include "utils/magic_buildinfo.h"
+
+char *MagicVersion = MAGIC_VERSION;
+char *MagicRevision = MAGIC_REVISION;
+char *MagicCommit = MAGIC_COMMIT;
+
+/*
+ * MAGIC_BUILDDATE may be disabled at configure time (--disable-magic-builddate)
+ * for reproducible builds; when it is, MagicCompileTime reports the empty string.
+ */
+#ifdef MAGIC_NO_BUILDDATE
+char *MagicCompileTime = "";
+#else
+char *MagicCompileTime = MAGIC_BUILDDATE;
+#endif

utils/magic.h

@@ -145,9 +145,10 @@ extern char *SysLibPath;	/* Path for finding system
 
 /* ------------ Globally-used strings. -------------------------------- */
 
-extern char *MagicVersion;
-extern char *MagicRevision;
-extern char *MagicCompileTime;
+/* Build-info globals (MagicVersion / MagicRevision / MagicCommit /
+ * MagicCompileTime) are defined in utils/buildinfo.c -- the one unit compiled
+ * with the version defines -- and declared here: */
+#include "utils/magic_buildinfo.h"
 extern char AbortMessage[];
 
 /* ------------ zlib (compression) support -------------------------------- */

utils/magic_buildinfo.h

@@ -0,0 +1,24 @@
+/*
+ * magic_buildinfo.h --
+ *
+ * Public declarations for the build-information globals: magic's version,
+ * revision, git commit, and compile date/time.
+ *
+ * These are the ONLY symbols initialized from the -DMAGIC_VERSION /
+ * -DMAGIC_REVISION / -DMAGIC_COMMIT / -DMAGIC_BUILDDATE command-line defines.
+ * They are defined in exactly one translation unit, utils/buildinfo.c, which is
+ * the only unit compiled with those defines.  Every other unit reads the values
+ * through these externs, so the volatile build-date/commit values stay off every
+ * other compile command line (which keeps ccache/sccache keys stable and the
+ * baked-in date/commit consistent across the whole binary).
+ */
+
+#ifndef _MAGIC_BUILDINFO_H
+#define _MAGIC_BUILDINFO_H
+
+extern char *MagicVersion;	/* e.g. "8.3"  (major.minor) */
+extern char *MagicRevision;	/* e.g. "670"  (patch/revision) */
+extern char *MagicCommit;	/* git commit SHA, or "" if unavailable */
+extern char *MagicCompileTime;	/* build date/time, or "" if disabled */
+
+#endif /* _MAGIC_BUILDINFO_H */