CPack: add CPACK_THREADS variable to control compression threads

This allows setting how many threads the compressor will use.
Currently only implemented for XZ when using system's lzma library.

Fixes: #21715

Source/cmArchiveWrite.h

@@ -54,7 +54,8 @@ public:
 
   /** Construct with output stream to which to write archive.  */
   cmArchiveWrite(std::ostream& os, Compress c = CompressNone,
-                 std::string const& format = "paxr", int compressionLevel = 0);
+                 std::string const& format = "paxr", int compressionLevel = 0,
+                 int numThreads = 1);
 
   ~cmArchiveWrite();