ENH: Allow empty endif() and such with CMake 2.4

This allows us to use empty endif() and similar block terminators when
building with CMake 2.4.  It is allowed by default with 2.6 already.

CMakeLists.txt

@@ -6,6 +6,8 @@ ENDIF(COMMAND CMAKE_POLICY)
 
 MARK_AS_ADVANCED(CMAKE_BACKWARDS_COMPATIBILITY)
 
+# Allow empty endif() and such with CMake 2.4.
+SET(CMAKE_ALLOW_LOOSE_LOOP_CONSTRUCTS 1)
 
 
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