Make INTERFACE determined properties readable in generator expressions.

The properties are evaluated as link-dependent interface properties when
evaluating the generator expressions.

Tests/CompatibleInterface/CMakeLists.txt

@@ -0,0 +1,28 @@
+
+cmake_minimum_required(VERSION 2.8)
+
+project(CompatibleInterface)
+
+add_library(iface1 empty.cpp)
+set_property(TARGET iface1 APPEND PROPERTY
+  COMPATIBLE_INTERFACE_BOOL
+    BOOL_PROP1
+    BOOL_PROP2
+    BOOL_PROP3
+)
+
+set_property(TARGET iface1 PROPERTY INTERFACE_BOOL_PROP1 ON)
+set_property(TARGET iface1 PROPERTY INTERFACE_BOOL_PROP2 ON)
+
+add_executable(CompatibleInterface main.cpp)
+target_link_libraries(CompatibleInterface iface1)
+
+set_property(TARGET CompatibleInterface PROPERTY BOOL_PROP2 ON)
+set_property(TARGET CompatibleInterface PROPERTY BOOL_PROP3 ON)
+
+target_compile_definitions(CompatibleInterface
+  PRIVATE
+    $<$<BOOL:$<TARGET_PROPERTY:BOOL_PROP1>>:BOOL_PROP1>
+    $<$<BOOL:$<TARGET_PROPERTY:BOOL_PROP2>>:BOOL_PROP2>
+    $<$<BOOL:$<TARGET_PROPERTY:BOOL_PROP3>>:BOOL_PROP3>
+)

Tests/CompatibleInterface/empty.cpp

@@ -0,0 +1 @@
+// no content

Tests/CompatibleInterface/main.cpp

@@ -0,0 +1,17 @@
+
+#ifndef BOOL_PROP1
+#error Expected BOOL_PROP1
+#endif
+
+#ifndef BOOL_PROP2
+#error Expected BOOL_PROP2
+#endif
+
+#ifndef BOOL_PROP3
+#error Expected BOOL_PROP3
+#endif
+
+int main(int argc, char **argv)
+{
+  return 0;
+}