build: autodetect ccache (configure --disable-ccache to opt out)

When ccache is on PATH, prefix CC/CXX with it so every compile goes through
ccache -- no Makefile change, since rules.mak already runs `${CC} ... -c` and
defs.mak substitutes @CC@/@CXX@.  Autodetected on by default; --disable-ccache
opts out.

Placed after the AC_PROG_C* checks so configure's own feature tests use the
plain compiler and only the build picks up the prefix.  Pairs with the
build-info isolation (utils/buildinfo.c): with the volatile MAGIC_BUILDDATE on
only that one unit, a full rebuild hits the cache for every other unit.

Measured (default Tcl build, all objects deleted then rebuilt): 323/324 hits
with the date enabled (only buildinfo.o misses), 324/324 with
--disable-magic-builddate.

Verified: default configure -> CC="ccache gcc", CXX="ccache g++";
--disable-ccache -> CC="gcc".

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

scripts/configure

@@ -691,6 +691,7 @@ ALLOCA
 EGREP
 GREP
 PERL
+CCACHE
 SED
 RANLIB
 INSTALL_DATA
@@ -760,6 +761,7 @@ SHELL'
 ac_subst_files=''
 ac_user_opts='
 enable_option_checking
+enable_ccache
 with_gnu_ld
 enable_assertions
 enable_debug
@@ -1425,6 +1427,7 @@ Optional Features:
   --disable-option-checking  ignore unrecognized --enable/--with options
   --disable-FEATURE       do not include FEATURE (same as --enable-FEATURE=no)
   --enable-FEATURE[=ARG]  include FEATURE [ARG=yes]
+  --disable-ccache             do not use ccache even if it is installed
   --enable-assertions          build with fatal assertions enabled
   --enable-debug               build with fatal debug enabled
   --disable-compression        disable file compression
@@ -4233,6 +4236,59 @@ $as_echo "$ac_cv_path_SED" >&6; }
   rm -f conftest.sed
 
 
+# Check whether --enable-ccache was given.
+if test "${enable_ccache+set}" = set; then :
+  enableval=$enable_ccache;
+else
+  enable_ccache=yes
+fi
+
+if test "x$enable_ccache" = "xyes"; then
+    # Extract the first word of "ccache", so it can be a program name with args.
+set dummy ccache; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_prog_CCACHE+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if test -n "$CCACHE"; then
+  ac_cv_prog_CCACHE="$CCACHE" # Let the user override the test.
+else
+as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_prog_CCACHE="ccache"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+fi
+fi
+CCACHE=$ac_cv_prog_CCACHE
+if test -n "$CCACHE"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $CCACHE" >&5
+$as_echo "$CCACHE" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+    if test "x$CCACHE" != "x"; then
+        CC="$CCACHE $CC"
+        test "x$CXX" != "x" && CXX="$CCACHE $CXX"
+        { $as_echo "$as_me:${as_lineno-$LINENO}: using ccache: CC=\"$CC\"" >&5
+$as_echo "$as_me: using ccache: CC=\"$CC\"" >&6;}
+    fi
+fi
+
 # Extract the first word of "perl", so it can be a program name with args.
 set dummy perl; ac_word=$2
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5

scripts/configure.in

@@ -29,6 +29,26 @@ AC_PROG_INSTALL
 AC_PROG_RANLIB
 AC_PROG_SED
 
+dnl Prefix the compilers with ccache when it is installed (autodetected; opt out
+dnl with --disable-ccache).  Done after the AC_PROG_C* checks above so those
+dnl feature tests run with the plain compiler; only the substituted @CC@/@CXX@
+dnl used by the build pick up the prefix, and rules.mak's `${CC} ... -c` recipe
+dnl then goes through ccache with no Makefile change.  This pairs with the
+dnl build-info isolation (utils/buildinfo.c): the volatile MAGIC_BUILDDATE lives
+dnl only on that one unit's command line, so a rebuild hits the cache for every
+dnl other unit instead of missing all of them.
+AC_ARG_ENABLE(ccache,
+[  --disable-ccache             do not use ccache even if it is installed],
+[], [enable_ccache=yes])
+if test "x$enable_ccache" = "xyes"; then
+    AC_CHECK_PROG(CCACHE, ccache, ccache)
+    if test "x$CCACHE" != "x"; then
+        CC="$CCACHE $CC"
+        test "x$CXX" != "x" && CXX="$CCACHE $CXX"
+        AC_MSG_NOTICE([using ccache: CC="$CC"])
+    fi
+fi
+
 AC_PATH_PROG(PERL, perl, no)
 AC_SUBST(PERL)