cmake: Don't rely on $<TARGET_PROPERTY:...> in CMake < 2.8.10.

cmake/macros/Interrogate.cmake

@@ -167,13 +167,28 @@ function(interrogate_sources target output database module)
     endforeach(source)
 
     # Interrogate also needs the include paths, so we'll extract them from the
-    # target using a generator expression.
-    # Note, the \t is a workaround for a CMake bug where using a plain space in
-    # a JOIN will cause it to be escaped. Tabs are not escaped and will
-    # separate correctly.
-    set(include_flags "-I$<JOIN:$<TARGET_PROPERTY:${target},INTERFACE_INCLUDE_DIRECTORIES>,\t-I>")
-    # The above must also be included when compiling the resulting _igate.cxx file:
-    include_directories("$<TARGET_PROPERTY:${target},INTERFACE_INCLUDE_DIRECTORIES>")
+    # target. Usually these are available via a generator expression, however
+    # versions of CMake before 2.8.10 don't support the TARGET_PROPERTY genex
+    # so we have to extract the property at configuration-time. Versions after
+    # 2.8.11 support CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE which ONLY makes
+    # this information available at generator-time (i.e. after
+    # configuration-time) so the generator expression is necessary then.
+    if(CMAKE_VERSION VERSION_LESS 2.8.10)
+      set(include_flags)
+      get_target_property(include_dirs "${target}" INTERFACE_INCLUDE_DIRECTORIES)
+      foreach(include_dir ${include_dirs})
+        list(APPEND include_flags "-I${include_dir}")
+      endforeach(include_dir)
+      # The above must also be included when compiling the resulting _igate.cxx file:
+      include_directories(${include_dirs})
+    else()
+      # Note, the \t is a workaround for a CMake bug where using a plain space in
+      # a JOIN will cause it to be escaped. Tabs are not escaped and will
+      # separate correctly.
+      set(include_flags "-I$<JOIN:$<TARGET_PROPERTY:${target},INTERFACE_INCLUDE_DIRECTORIES>,\t-I>")
+      # The above must also be included when compiling the resulting _igate.cxx file:
+      include_directories("$<TARGET_PROPERTY:${target},INTERFACE_INCLUDE_DIRECTORIES>")
+    endif()
 
     # Get the compiler definition flags. These must be passed to Interrogate
     # in the same way that they are passed to the compiler so that Interrogate