ExternalData: Add option to disable use of symlinks

Add an ExternalData_NO_SYMLINKS to enable use of copies instead of
symlinks to populate the real data files behind a DATA{} reference.
This will be useful on UNIX-like systems when the underlying filesystem
does not actually support symbolic links.

Suggested-by: Matt McCormick <matt.mccormick@kitware.com>

Tests/Module/ExternalData/CMakeLists.txt

@@ -53,3 +53,4 @@ ExternalData_Add_Target(Data1)
 add_subdirectory(Data2)
 add_subdirectory(Data3)
 add_subdirectory(Data4)
+add_subdirectory(DataNoSymlinks)

Tests/Module/ExternalData/DataNoSymlinks/CMakeLists.txt

@@ -0,0 +1,8 @@
+set(ExternalData_NO_SYMLINKS 1)
+ExternalData_Add_Test(DataNoSymlinks
+  NAME DataNoSymlinksCheck
+  COMMAND ${CMAKE_COMMAND}
+    -D Data=DATA{Data.dat}
+    -P ${CMAKE_CURRENT_SOURCE_DIR}/DataNoSymlinksCheck.cmake
+  )
+ExternalData_Add_Target(DataNoSymlinks)

Tests/Module/ExternalData/DataNoSymlinks/Data.dat.md5

@@ -0,0 +1 @@
+8c018830e3efa5caf3c7415028335a57

Tests/Module/ExternalData/DataNoSymlinks/DataNoSymlinksCheck.cmake

@@ -0,0 +1,6 @@
+if(NOT EXISTS "${Data}")
+  message(SEND_ERROR "Input file:\n  ${Data}\ndoes not exist!")
+endif()
+if(IS_SYMLINK "${Data}")
+  message(SEND_ERROR "Input file:\n  ${Data}\nis a symlink but should not be!")
+endif()