ExternalProject: Fix path to cmake in verify script

In commit v3.0.0-rc1~103^2~3 (ExternalProject: Reattempt download when
verification fails, 2014-01-15) a reference to ${CMAKE_COMMAND} was
added to generate a reference to the CMake command in a cmake script.
Escape the '$' so that the literal variable reference appears in the
script instead of writing the path to the current cmake.  This is
necessary when the path to CMake contains spaces or other characters
special to CMake syntax.

Modules/ExternalProject.cmake

@@ -605,7 +605,7 @@ does not match expected value
 Retrying download.
 \")
     file(REMOVE \"\${file}\")
-    execute_process(COMMAND ${CMAKE_COMMAND} -P \"${download_script}\")
+    execute_process(COMMAND \${CMAKE_COMMAND} -P \"${download_script}\")
   endif()
 endwhile()