build: per-file automatic dependencies (.deps), retire the Depend/sed machinery

Replace the monolithic per-directory `Depend` file -- generated by a separate,
serial `make depend` pass (one `gcc -MM` over all sources) and post-processed by
sed -- with automake-style per-file dependency fragments produced as a side
effect of each compile.

rules.mak's %.o rule now adds, when the compiler supports it,
  -MMD -MP -MF .deps/<stem>.d
so every object records the (non-system) headers it used.  Those fragments are
`-include`d best-effort: absent on a first build, present and correct after.
Benefits:
  * self-maintaining -- no explicit depend step; deps refresh on every compile;
  * parallel -- generated during the (already parallel) compile, no barrier;
  * -MMD already drops system headers (what the sed did) and -MP tolerates a
    removed/renamed header, so the sed post-processing is gone;
  * fixes out-of-tree incremental header tracking: with an absolute srcdir the
    old sed stripped the now-absolute *local* header paths too, silently losing
    them; the .d files keep them.

configure probes `-MMD -MP` (AUTODEP_FLAGS; empty and best-effort-skipped if the
compiler cannot).  clean/distclean remove .deps (rm -r); .gitignore replaces the
obsolete */Depend with .deps/.  `make depend` is kept as a no-op for callers that
still invoke it (e.g. the WASM build script).

Verified out-of-tree (--disable-ccache): 324 .d files, 0 Depend files, a header
touch recompiles its dependents, no-op rebuild is idle, clean clears .deps.  (The
top-level depend phase still runs here as a no-op; removed next.)

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

scripts/configure

@@ -636,7 +636,7 @@ MSED
 MCPP
 LD_RUN_PATH
 SHLIB_CFLAGS
-DEPEND_FLAG
+AUTODEP_FLAGS
 gr_hprog
 gr_hsrcs
 gr_srcs
@@ -6349,7 +6349,7 @@ extra_defs="$extra_defs -DCAD_DIR=\\\"\${LIBDIR}\\\" -DBIN_DIR=\\\"\${BINDIR}\\\
 X_LIBS=
 X_CFLAGS=
 INC_SPECS=
-DEPEND_FLAG=
+AUTODEP_FLAGS=
 SHLIB_CFLAGS=""
 LD_RUN_PATH=""
 WISH_EXE=""
@@ -9435,9 +9435,36 @@ fi
 
 fi
 
-if test "$GCC" = "yes" ; then
-   DEPEND_FLAG="-MM"
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC generates dependencies with -MMD -MP" >&5
+$as_echo_n "checking whether $CC generates dependencies with -MMD -MP... " >&6; }
+magic_save_cflags="$CFLAGS"
+CFLAGS="$CFLAGS -MMD -MP -MF conftest.autodep.d"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  if test -f conftest.autodep.d ; then
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }; AUTODEP_FLAGS="-MMD -MP"
+     else
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: no (produced no .d)" >&5
+$as_echo "no (produced no .d)" >&6; }
+     fi
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
 fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+rm -f conftest.autodep.d
+CFLAGS="$magic_save_cflags"
 
 
 

scripts/configure.in

@@ -370,7 +370,7 @@ extra_defs="$extra_defs -DCAD_DIR=\\\"\${LIBDIR}\\\" -DBIN_DIR=\\\"\${BINDIR}\\\
 X_LIBS=
 X_CFLAGS=
 INC_SPECS=
-DEPEND_FLAG=
+AUTODEP_FLAGS=
 SHLIB_CFLAGS=""
 LD_RUN_PATH=""
 WISH_EXE=""
@@ -1987,9 +1987,25 @@ if test $usingTcl ; then
 
 fi
 
-if test "$GCC" = "yes" ; then
-   DEPEND_FLAG="-MM"
-fi
+dnl Automatic per-file dependency generation.  Compilers that support -MMD -MP
+dnl write a .deps/<stem>.d listing the (non-system) headers a source used, as a
+dnl side effect of compiling it -- replacing the old serial "make depend" pass
+dnl and its sed post-processing, and parallelising with the compile.  Best
+dnl effort: empty if unsupported, and the build proceeds without incremental
+dnl header tracking.  Probed (not assumed from $GCC) so clang and others are
+dnl covered only when they actually produce the file.
+AC_MSG_CHECKING([whether $CC generates dependencies with -MMD -MP])
+magic_save_cflags="$CFLAGS"
+CFLAGS="$CFLAGS -MMD -MP -MF conftest.autodep.d"
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [])],
+    [if test -f conftest.autodep.d ; then
+        AC_MSG_RESULT([yes]); AUTODEP_FLAGS="-MMD -MP"
+     else
+        AC_MSG_RESULT([no (produced no .d)])
+     fi],
+    [AC_MSG_RESULT([no])])
+rm -f conftest.autodep.d
+CFLAGS="$magic_save_cflags"
 
 
 dnl Substitute all variables
@@ -2032,7 +2048,7 @@ AC_SUBST(CFLAGS)
 AC_SUBST(CPPFLAGS)
 AC_SUBST(CXX)
 AC_SUBST(CXXFLAGS)
-AC_SUBST(DEPEND_FLAG)
+AC_SUBST(AUTODEP_FLAGS)
 AC_SUBST(SHLIB_CFLAGS)
 AC_SUBST(LDFLAGS)
 AC_SUBST(LD_RUN_PATH)

scripts/defs.mak.in

@@ -200,8 +200,10 @@ LDFLAGS                = @LDFLAGS@
 READLINE_DEFS          = @rl_defs@
 READLINE_LIBS          = @rl_libs@
 
-DEPEND_FILE	       = Depend
-DEPEND_FLAG	       = @DEPEND_FLAG@
+# Per-file dependency flags ("-MMD -MP" when the compiler supports it, else
+# empty); used by the %.o rule in rules.mak to write .deps/<stem>.d as a compile
+# side effect.  Replaces the old serial "make depend" / Depend-file machinery.
+AUTODEP_FLAGS	       = @AUTODEP_FLAGS@
 EXEEXT		       = @EXEEXT@
 
 GR_CFLAGS              = @X_CFLAGS@ @gr_cflags@
@@ -214,12 +216,11 @@ GR_HELPER_PROG         = @gr_hprog@
 OA		       = @OA@
 OA_LIBS		       = @OA_LIBS@
 
-DEPSRCS	  = ${SRCS}
 C_OBJS    = ${SRCS:.c=.o}
 CXX_OBJS  = ${CXXSRCS:.cpp=.o}
 OBJS      = ${CXX_OBJS} ${C_OBJS}
 LIB_OBJS  = ${LIB_SRCS:.c=.o}
-CLEANS    = Depend ${OBJS} ${LIB_OBJS} lib${MODULE}.a lib${MODULE}.o ${MODULE}
+CLEANS    = ${OBJS} ${LIB_OBJS} lib${MODULE}.a lib${MODULE}.o ${MODULE}
 
 # allows Makefile to be selective
 MAKE_TCL           = @MAKE_TCL@