ci: Add more checks to ci_verify_version.sh; refactor

In addition to png.h, configure.ac and CMakeLists.txt, the script
ci_verify_version.sh is now able to verify libpng-config-head.in also.

For the benefit of readability, the old script ci_shellify.sh has been
split into smaller, independent scriptlets: libexec/ci_shellify_*.sh.

The linting script ci_lint.sh has been updated as needed.

ci/libexec/ci_shellify_autoconf.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+set -o errexit -o pipefail -o posix
+
+# Copyright (c) 2019-2025 Cosmin Truta.
+#
+# Use, modification and distribution are subject to the MIT License.
+# Please see the accompanying file LICENSE_MIT.txt
+#
+# SPDX-License-Identifier: MIT
+
+# shellcheck source=ci/lib/ci.lib.sh
+source "$(dirname "$0")/../lib/ci.lib.sh"
+
+function ci_shellify_autoconf {
+    # Convert autoconf (M4) text, specifically originating
+    # from configure.ac, to shell scripting text.
+    # Select only the easy-to-parse definitions of PNGLIB_*.
+    sed -n -e '/^ *PNGLIB_[^ ]*=[$"0-9A-Za-z_]/ p' |
+        sed -e 's/^ *PNG\([0-9A-Za-z_]*\)=\([^# ]*\).*$/PNG\1=\2/' \
+            -e 's/^\([^ ]*=[^ ]*\).*$/export \1;/'
+}
+
+function usage {
+    echo "usage: $CI_SCRIPT_NAME [<options>] configure.ac"
+    echo "options: -?|-h|--help"
+    exit "${@:-0}"
+}
+
+function main {
+    local opt
+    while getopts ":" opt
+    do
+        # This ain't a while-loop. It only pretends to be.
+        [[ $1 == -[?h]* || $1 == --help || $1 == --help=* ]] && usage 0
+        ci_err "unknown option: '$1'"
+    done
+    shift $((OPTIND - 1))
+    [[ $# -eq 0 ]] && usage 2
+    [[ $# -eq 1 ]] || ci_err "too many operands"
+    # And... go!
+    test -e "$1" || ci_err "no such file: '$1'"
+    [[ $(basename -- "$1") == configure.ac ]] || {
+        ci_err "incorrect operand: '$1' (expecting: 'configure.ac')"
+    }
+    ci_shellify_autoconf <"$1"
+}
+
+main "$@"