ExternalProject: Avoid bleed-through output when logging.

Unset VS_UNICODE_OUTPUT when executing a command whose output
is being logged to a file. Previously, running Microsoft tools
in sub-processes of Visual Studio would send their output to
the Visual Studio output pipe named by this environment variable.

Unsetting it forces the output back to the normal stdout and stderr
channels where cmake can intercept it and direct it to the
appropriate log files.

Modules/ExternalProject.cmake

@@ -711,7 +711,7 @@ endif()
         set(sep ";")
       endif()
     endforeach()
-    set(code "${code}set(command \"${cmd}\")${code_execute_process}")
+    set(code "set(ENV{VS_UNICODE_OUTPUT} \"\")\n${code}set(command \"${cmd}\")${code_execute_process}")
     file(WRITE ${stamp_dir}/${name}-${step}-impl.cmake "${code}")
     set(command ${CMAKE_COMMAND} "-Dmake=\${make}" "-Dconfig=\${config}" -P ${stamp_dir}/${name}-${step}-impl.cmake)
   endif()
@@ -721,6 +721,7 @@ endif()
   set(logbase ${stamp_dir}/${name}-${step})
   file(WRITE ${script} "
 ${code_cygpath_make}
+set(ENV{VS_UNICODE_OUTPUT} \"\")
 set(command \"${command}\")
 execute_process(
   COMMAND \${command}