ci: Add script to install macOS SDK version 15.5

With Xcode 16.4, run

    env SDKROOT=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk \
    .gitlab/ci/repackage/macos.sh

and host `MacOSX15.5.sdk.tar.bz2` ourselves.

.gitlab/.gitignore

@@ -8,6 +8,7 @@
 /innosetup
 /jom
 /llvm*
+/MacOS*
 /mingw
 /msvc*
 /ninja*

.gitlab/ci/macos-env.sh

@@ -0,0 +1,2 @@
+.gitlab/ci/macos.sh
+export SDKROOT="$PWD/.gitlab/MacOSX.sdk"

.gitlab/ci/macos.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+set -e
+
+# This URL is only visible inside of Kitware's network.
+baseurl="https://cmake.org/files/dependencies/internal/macos"
+
+case "$(uname -s)-$(uname -m)" in
+    Darwin-*)
+        shatool="shasum -a 256"
+        sha256sum="427612880d6c40bcef2b0ecb39d92b057ee7a43ec3552fbd4449859991eb1cc6"
+        tarball="MacOSX15.5.sdk.tar.bz2"
+        ;;
+    *)
+        echo "Unrecognized platform $(uname -s)-$(uname -m)"
+        exit 1
+        ;;
+esac
+readonly shatool
+readonly sha256sum
+
+cd .gitlab
+
+echo "$sha256sum  $tarball" > macos.sha256sum
+curl -OL "$baseurl/$tarball"
+$shatool --check macos.sha256sum
+tar xjf "$tarball"
+rm "$tarball" macos.sha256sum