ExternalProject: Avoid infinite loop on file download hash mismatch

When the loop was added by commit v3.0.0-rc1~103^2~3 (ExternalProject:
Reattempt download when verification fails, 2015-01-15), we forgot to
actually increment the loop counter.  Add the increment line.

Modules/ExternalProject.cmake

@@ -607,6 +607,7 @@ Retrying download.
     file(REMOVE \"\${file}\")
     execute_process(COMMAND \${CMAKE_COMMAND} -P \"${download_script}\")
   endif()
+  math(EXPR attempt \"\${attempt} + 1\")
 endwhile()
 
 if(\${succeeded})