ci: Add script to repackage Pelles C compilers

Pelles C compilers are provided only via installer executables.
Add a script to repackage the parts we need for CMake's CI jobs.

Issue: #21536

.gitlab/ci/repackage/pellesc.ps1

@@ -0,0 +1,74 @@
+# Pelles C compilers are available only via installers.
+# Run an installer and repackage the installation directory.
+
+# From the Pelles C download page, download "setup.exe".
+# Run this script passing "setup.exe" followed by the version number.
+
+#Requires -RunAsAdministrator
+
+param (
+  [Parameter(Mandatory=$true)]
+  [string]$installer,
+  [Parameter(Mandatory=$true)]
+  [string]$version,
+  [string]$revision = "1",
+  [string]$basedir = "c:\PellesC"
+  )
+
+$erroractionpreference = "stop"
+
+Add-Type -AssemblyName System.IO.Compression.FileSystem
+
+$installer_file = Get-Item $installer
+$installer_name = $installer_file.Name
+$package_name = "pellesc-$version-$revision"
+$package_dir = New-Item -Force -ItemType Directory -Path "$basedir\$package_name"
+if (-not $package_dir) {
+  Write-Host "Failed to create package install dir."
+  Exit 1
+}
+
+Write-Host "Installing to: $package_dir"
+
+# The installer treats everything after /D= as the destination.
+# Start-Process adds a trailing space, so use "cmd" instead.
+& "$env:ComSpec" /c start /wait "$installer_file" /S /D=$package_dir
+Write-Host ""
+Remove-Item "$package_dir/uninst.exe" -Force
+
+# Convert environment scripts to templates.
+$bats = @(
+  "bin\povars32.bat"
+  "bin\povars64.bat"
+  )
+foreach ($p in $bats) {
+  $bat = Get-Content -Path "$package_dir\$p" -Raw
+  $bat = $bat -Replace "PellesCDir=.*","PellesCDir=@PellesCDir@"
+  $bat | Set-Content -Path "$package_dir\$p.in"
+  Remove-Item "$package_dir\$p" -Force
+}
+
+@"
+This was repackaged from an installation by
+
+  Pelles C $version "$installer_name"
+
+using CMake's ".gitlab/ci/repackage/pellesc.ps1" script.
+
+Copy or rename the environment script templates in the "bin" directory:
+
+  povars32.bat.in => povars32.bat
+  povars64.bat.in => povars64.bat
+
+and replace the "@PellesCDir@" placeholders with
+
+  C:\path\to\this\directory
+
+Then use "povars32.bat" or "povars64.bat" to establish an environment.
+
+"@ | Add-Content -NoNewline "$package_dir/README.txt"
+
+Write-Host "Repackaging to: $package_name.zip"
+$compressionLevel = [System.IO.Compression.CompressionLevel]::Optimal
+$includeBaseDirectory = $true
+[System.IO.Compression.ZipFile]::CreateFromDirectory("$package_dir", "$package_name.zip", $compressionLevel, $includeBaseDirectory)