build: replace distributed gzipped sources with zstandard

zstd compressed sources are less than half the size
of gzip coompressed sources, so are a better secondary
distribution format in addition to xz.

* README-prereq: Remove gzip, Add zstd.
* bootstrap.conf (buildreq): Likewise.
Bump automake version check to 1.16.2 (released 2020).
* configure.ac (AM_INIT_AUTOMAKE): Disable gz, add dist-zstd.
Bump automake version requirement to 1.16.2.
* cfg.mk: Use compression level 18, and multi-threading.
* NEWS: Mention the change.

NEWS

@@ -17,6 +17,11 @@ GNU coreutils NEWS                                    -*- outline -*-
 
   'od' allocates half the memory when -v,--output-duplicates is specified.
 
+** Build-related
+
+  gzip-compressed tarballs are no longer distributed,
+  instead being replaced with zstandard-compressed tarballs.
+
 
 * Noteworthy changes in release 9.12 (2026-09-14) [stable]
 

README-prereq

@@ -9,7 +9,6 @@ as well as by 'make'.  They include:
 - Gettext    <https://www.gnu.org/software/gettext/>
 - Git        <https://git-scm.com/>
 - Gperf      <https://www.gnu.org/software/gperf/>
-- Gzip       <https://www.gnu.org/software/gzip/>
 - Help2man   <https://www.gnu.org/software/help2man/>
 - M4         <https://www.gnu.org/software/m4/>
 - Make       <https://www.gnu.org/software/make/>
@@ -18,6 +17,7 @@ as well as by 'make'.  They include:
 - Texinfo    <https://www.gnu.org/software/texinfo/>
 - Wget       <https://www.gnu.org/software/wget/>
 - XZ Utils   <https://tukaani.org/xz/>
+- Zstandard  <https://github.com/facebook/zstd>
 
 It is generally better to use official packages for your system.
 If a package is not officially available you can build it from source

bootstrap.conf

@@ -390,13 +390,12 @@ gnulib_tool_option_extras="--tests-base=gnulib-tests --with-tests --symlink\
 # Build prerequisites
 buildreq="\
 autoconf   2.64
-automake   1.11.2
+automake   1.16.2
 autopoint  0.19.2
 bison      -
 gettext    0.19.2
 git        1.5.5
 gperf      -
-gzip       -
 m4         -
 makeinfo   6.1
 texi2pdf   6.1
@@ -405,6 +404,7 @@ perl       5.5
 tar        -
 wget       -
 xz         -
+zstd       -
 "
 
 bootstrap_post_import_hook ()

cfg.mk

@@ -51,6 +51,15 @@ export VERBOSE = yes
 # 4914152 9e
 export XZ_OPT = -8e
 
+# -T0 uses all CPUs,
+# -18 output is only 2% larger but compresses twice as fast as -19
+# Tested with:
+#   for t in '' '-T0'; do for l in $(seq 15 19); do
+#     echo == $t -$l ==; env time -f 'elapsed=%E CPU=%Us Mem=%MKB' \
+#      zstd -c $t -$l < coreutils-9.12.tar | wc -c
+#   done; done
+export ZSTD_OPT = -T0 -18
+
 old_NEWS_hash = 078ebe66312fe06ceac192cd5a8423ec
 
 # Add an exemption for sc_makefile_at_at_check.

configure.ac

@@ -35,7 +35,8 @@ AC_CONFIG_SRCDIR([src/ls.c])
 AC_CONFIG_AUX_DIR([build-aux])
 AC_CONFIG_HEADERS([lib/config.h:lib/config.hin])
 
-AM_INIT_AUTOMAKE([1.11.2 dist-xz color-tests parallel-tests subdir-objects])
+AM_INIT_AUTOMAKE([1.16.2 no-dist-gzip dist-xz dist-zstd color-tests
+                  parallel-tests subdir-objects])
 AM_SILENT_RULES([yes]) # make --enable-silent-rules the default.
 
 dnl POSIXCHECK is worthwhile for maintainers, but adds several seconds