Use RENAME, not COPY, to extract .tar.gz files.

  Use ABSOLUTE file name for the RENAME operation.
  (Thanks to Marcus for figuring out that it doesn't
   work on Linux without the ABSOLUTE bit.)

Modules/ExternalProject.cmake

@@ -243,7 +243,7 @@ message(STATUS \"downloading... done\")
 endfunction(_ep_write_downloadfile_script)
 
 
-function(_ep_write_extractfile_script script_filename filename tmp directory)
+function(_ep_write_extractfile_script script_filename filename directory)
   set(args "")
 
   if(filename MATCHES ".tar$")
@@ -267,7 +267,6 @@ function(_ep_write_extractfile_script script_filename filename tmp directory)
 "# Make file names absolute:
 #
 get_filename_component(filename \"${filename}\" ABSOLUTE)
-get_filename_component(tmp \"${tmp}\" ABSOLUTE)
 get_filename_component(directory \"${directory}\" ABSOLUTE)
 
 message(STATUS \"extracting...
@@ -276,11 +275,11 @@ message(STATUS \"extracting...
 
 # Prepare a space for extracting:
 #
-set(i 1)
-while(EXISTS \"\${tmp}/extract\${i}\")
+set(i 1234)
+while(EXISTS \"\${directory}/../ex\${i}\")
   math(EXPR i \"\${i} + 1\")
 endwhile()
-set(ut_dir \"\${tmp}/extract\${i}\")
+set(ut_dir \"\${directory}/../ex\${i}\")
 file(MAKE_DIRECTORY \"\${ut_dir}\")
 
 # Extract it:
@@ -305,10 +304,12 @@ if(NOT n EQUAL 1 OR NOT IS_DIRECTORY \"\${contents}\")
   set(contents \"\${ut_dir}\")
 endif()
 
-# Copy \"the one\" directory to the final directory:
+# Move \"the one\" directory to the final directory:
 #
-message(STATUS \"extracting... [copy]\")
-file(COPY \"\${contents}/\" DESTINATION \${directory})
+message(STATUS \"extracting... [rename]\")
+file(REMOVE_RECURSE \${directory})
+get_filename_component(contents \${contents} ABSOLUTE)
+file(RENAME \${contents} \${directory})
 
 # Clean up:
 #
@@ -710,7 +711,7 @@ function(_ep_add_download_command name)
         set(comment "Performing download step (extract) for '${name}'")
       endif()
       # TODO: Support other archive formats.
-      _ep_write_extractfile_script("${stamp_dir}/extract-${name}.cmake" "${file}" "${tmp_dir}" "${source_dir}")
+      _ep_write_extractfile_script("${stamp_dir}/extract-${name}.cmake" "${file}" "${source_dir}")
       list(APPEND cmd ${CMAKE_COMMAND} -P ${stamp_dir}/extract-${name}.cmake)
     endif()
   else()