ExternalProject: Allow dependencies on INTERFACE libraries

Respect INTERFACE library property whitelist.  Check that a target has
type "UTILITY" before querying other properties.

Tests/ExternalProjectSubdir/CMakeLists.txt

@@ -2,6 +2,19 @@ cmake_minimum_required(VERSION 3.2)
 project(ExternalProjectSubdir NONE)
 include(ExternalProject)
 
+# Remove the custom target output to be sure it runs in an
+# incremental test.  Skip this on VS 6 because it sometimes
+# re-runs CMake after the custom command runs.
+if(NOT CMAKE_GENERATOR STREQUAL "Visual Studio 6")
+  file(REMOVE ${CMAKE_CURRENT_BINARY_DIR}/PreSubdir1.txt)
+endif()
+
+add_custom_target(PreSubdir1
+  COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_CURRENT_BINARY_DIR}/PreSubdir1.txt
+  )
+add_library(PreSubdir1Interface INTERFACE)
+add_dependencies(PreSubdir1Interface PreSubdir1)
+
 ExternalProject_Add(Subdir1
   SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/Subdir1
   BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/Subdir1
@@ -11,4 +24,6 @@ ExternalProject_Add(Subdir1
 
   BUILD_COMMAND ""
   INSTALL_COMMAND ""
+
+  DEPENDS PreSubdir1Interface
   )

Tests/ExternalProjectSubdir/Subdir1/CMakeLists.txt

@@ -8,3 +8,7 @@ endif()
 if(NOT "${GENEX_VAR}" STREQUAL "GENEX_VALUE")
   message(SEND_ERROR "GENEX_VAR != 'GENEX_VALUE'")
 endif()
+
+if(NOT EXISTS "${CMAKE_CURRENT_BINARY_DIR}/../PreSubdir1.txt")
+  message(SEND_ERROR "../PreSubdir1.txt not provided!")
+endif()