ExternalProject: Ensure _ep_set_directories records cmake paths

Modules/ExternalProject.cmake

@@ -1674,6 +1674,7 @@ function(_ep_set_directories name)
     endif()
   endif()
   if(prefix)
+    file(TO_CMAKE_PATH "${prefix}" prefix)
     set(tmp_default "${prefix}/tmp")
     set(download_default "${prefix}/src")
     set(source_default "${prefix}/src/${name}")
@@ -1681,6 +1682,7 @@ function(_ep_set_directories name)
     set(stamp_default "${prefix}/src/${name}-stamp")
     set(install_default "${prefix}")
   else()
+    file(TO_CMAKE_PATH "${base}" base)
     set(tmp_default "${base}/tmp/${name}")
     set(download_default "${base}/Download/${name}")
     set(source_default "${base}/Source/${name}")
@@ -1709,6 +1711,7 @@ function(_ep_set_directories name)
     if(NOT IS_ABSOLUTE "${${var}_dir}")
       get_filename_component(${var}_dir "${top}/${${var}_dir}" ABSOLUTE)
     endif()
+    file(TO_CMAKE_PATH "${${var}_dir}" ${var}_dir)
     set_property(TARGET ${name} PROPERTY _EP_${VAR}_DIR "${${var}_dir}")
   endforeach()
 
@@ -1720,6 +1723,7 @@ function(_ep_set_directories name)
   if(NOT IS_ABSOLUTE "${log_dir}")
     get_filename_component(log_dir "${top}/${log_dir}" ABSOLUTE)
   endif()
+  file(TO_CMAKE_PATH "${log_dir}" log_dir)
   set_property(TARGET ${name} PROPERTY _EP_LOG_DIR "${log_dir}")
 
   get_property(source_subdir TARGET ${name} PROPERTY _EP_SOURCE_SUBDIR)
@@ -1731,6 +1735,7 @@ function(_ep_set_directories name)
   else()
     # Prefix with a slash so that when appended to the source directory, it
     # behaves as expected.
+    file(TO_CMAKE_PATH "${source_subdir}" source_subdir)
     set_property(TARGET ${name} PROPERTY _EP_SOURCE_SUBDIR "/${source_subdir}")
   endif()
   if(build_in_source)