build: generate <magic/autoconf/config.h> from configure

Emit an autoconf config header carrying every autoconf define -- at least
all the -D options that appear on the compiler command lines, plus
anything else configure knows -- available to the tree as
    #include <magic/autoconf/config.h>

It is built directly from confdefs.h just before AC_OUTPUT (which removes
that file), NOT via AC_CONFIG_HEADERS.  This is deliberate:

  * AC_CONFIG_HEADERS would rewrite @DEFS@ to -DHAVE_CONFIG_H and move the
    defines into the header, changing every compiler command line and
    breaking the build until sources #include it.  Building from confdefs.h
    leaves @DEFS@ (and the whole build) exactly as-is, so the header is a
    pure additional artifact with no consumers yet.
  * confdefs.h already holds every AC_DEFINE, including the many that have
    no autoheader template (CALMA_MODULE, HAVE_READLINE, ...), which
    AC_CONFIG_HEADERS/autoheader would have dropped.

Location mirrors the generated database.h (defs.mak GENINC): magic/autoconf/
config.h at the build top when in-tree (build dir == source dir), else
include/magic/autoconf/config.h so the clean -I include/ rule finds it and
the raw build dir stays off the search path.  The directory is created on
demand and the header is rewritten on every ./configure.  No defs.mak.in
change is needed -- the existing -I${GENINC} already resolves it.

.gitignore: ignore the in-tree magic/autoconf/config.h (out-of-tree it is
under the build dir and already ignored).

Verified: out-of-tree writes include/magic/autoconf/config.h, in-tree
writes magic/autoconf/config.h; both contain the command-line defines
(SIZEOF_VOID_P, CALMA_MODULE, HAVE_*, ...); #include <magic/autoconf/
config.h> resolves via -I${GENINC} from a subdir; @DEFS@ is unchanged
(-DHAVE_CONFIG_H absent) and modules still build.

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

.gitignore

@@ -22,6 +22,9 @@ reconfigure.sh
 */Depend*.tmp
 .deps/
 database/database.h
+# autoconf config header, generated by ./configure (in-tree location; out-of-tree
+# it lives under the build dir's include/ and is covered by the build-dir rules).
+magic/autoconf/config.h
 
 # Editor / OS cruft
 .*.swp

scripts/configure

@@ -9398,6 +9398,24 @@ ac_config_files="$ac_config_files scripts/makedbh:scripts/makedbh.in"
 
 ac_config_files="$ac_config_files rules.mak:rules.mak"
 
+
+if test "`cd $srcdir && pwd`" = "`pwd`" ; then
+  magic_config_header=magic/autoconf/config.h
+else
+  magic_config_header=include/magic/autoconf/config.h
+fi
+mkdir -p "`dirname $magic_config_header`"
+{
+  echo "/* $magic_config_header -- generated by configure; DO NOT EDIT. */"
+  echo "/* Every autoconf define (>= all -D options on the compiler lines). */"
+  echo "#ifndef _MAGIC_AUTOCONF_CONFIG_H"
+  echo "#define _MAGIC_AUTOCONF_CONFIG_H 1"
+  grep '^#define ' confdefs.h | sort -u
+  echo "#endif /* _MAGIC_AUTOCONF_CONFIG_H */"
+} > "$magic_config_header"
+{ $as_echo "$as_me:${as_lineno-$LINENO}: created config header $magic_config_header" >&5
+$as_echo "$as_me: created config header $magic_config_header" >&6;}
+
 cat >confcache <<\_ACEOF
 # This file is a shell script that caches the results of configure
 # tests run on this system so they can be shared between configure

scripts/configure.in

@@ -2002,6 +2002,38 @@ dnl Copy rules.mak into the build top (it has no @...@ substitutions; this just
 dnl places it next to the generated defs.mak so a single ${MAGICDIR} resolves
 dnl both `include`s in every subdir Makefile out-of-tree).
 AC_CONFIG_FILES([rules.mak:rules.mak])
+
+dnl Emit an autoconf config header, available to the tree as
+dnl   #include <magic/autoconf/config.h>
+dnl It carries every autoconf define (i.e. at least all the -D options that
+dnl appear on the compiler command lines, plus anything else configure knows).
+dnl We build it here, from confdefs.h, rather than via AC_CONFIG_HEADERS on
+dnl purpose: that keeps @DEFS@ (hence the command lines and the whole build)
+dnl exactly as-is, so the header is a pure additional artifact with no consumers
+dnl yet.  confdefs.h holds every AC_DEFINE and is still present at this point
+dnl (AC_OUTPUT removes it), so it must run before AC_OUTPUT.
+dnl
+dnl Location mirrors the generated database.h (see defs.mak GENINC): at the
+dnl build top when in-tree (build dir == source dir), else under a clean
+dnl include/ subdir so the -I include/ rule finds <magic/autoconf/config.h> and
+dnl the raw build dir is never on the search path.  Regenerated on every
+dnl ./configure; the directory is created on demand.
+if test "`cd $srcdir && pwd`" = "`pwd`" ; then
+  magic_config_header=magic/autoconf/config.h
+else
+  magic_config_header=include/magic/autoconf/config.h
+fi
+mkdir -p "`dirname $magic_config_header`"
+{
+  echo "/* $magic_config_header -- generated by configure; DO NOT EDIT. */"
+  echo "/* Every autoconf define (>= all -D options on the compiler lines). */"
+  echo "#ifndef _MAGIC_AUTOCONF_CONFIG_H"
+  echo "#define _MAGIC_AUTOCONF_CONFIG_H 1"
+  grep '^#define ' confdefs.h | sort -u
+  echo "#endif /* _MAGIC_AUTOCONF_CONFIG_H */"
+} > "$magic_config_header"
+AC_MSG_NOTICE([created config header $magic_config_header])
+
 AC_OUTPUT
 
 dnl