From fdce782b61c6a1501b6ea2459c659d9a42bae59b Mon Sep 17 00:00:00 2001
From: Ben Boeckel <ben.boeckel@kitware.com>
Date: Tue, 18 Oct 2016 12:50:46 -0400
Subject: [PATCH] ExternalProject: error out only if the property is unset

Property values may be empty; instead make sure they exist and allow for
empty values.
---
 Modules/ExternalProject.cmake | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Modules/ExternalProject.cmake b/Modules/ExternalProject.cmake
index bbeeeed43..e3bdc6daa 100644
--- a/Modules/ExternalProject.cmake
+++ b/Modules/ExternalProject.cmake
@@ -1184,10 +1184,11 @@ endfunction()
 function(ExternalProject_Get_Property name)
   foreach(var ${ARGN})
     string(TOUPPER "${var}" VAR)
-    get_property(${var} TARGET ${name} PROPERTY _EP_${VAR})
-    if(NOT ${var})
+    get_property(is_set TARGET ${name} PROPERTY _EP_${VAR} SET)
+    if(NOT is_set)
       message(FATAL_ERROR "External project \"${name}\" has no ${var}")
     endif()
+    get_property(${var} TARGET ${name} PROPERTY _EP_${VAR})
     set(${var} "${${var}}" PARENT_SCOPE)
   endforeach()
 endfunction()