CMake: add target-level dependencies for conflict-set-object

The custom command that links conflict-set.o depends on
$<TARGET_OBJECTS:conflict-set-object>, but that generator expression
does not create a target-level dependency.  With the Unix Makefiles
generator, parallel builds can start building the consuming libraries
before the object library's build rule is available, producing:

  gmake[3]: *** No rule to make target
  'CMakeFiles/conflict-set-object.dir/ConflictSet.cpp.o', needed by
  'conflict-set.o'.  Stop.

Add add_dependencies() so that conflict-set and conflict-set-static
cannot build until conflict-set-object has produced its object files.

Closes #47

CMakeLists.txt

@@ -139,6 +139,7 @@ add_custom_command(
   COMMAND_EXPAND_LISTS)
 
 add_library(${PROJECT_NAME} SHARED ${CMAKE_BINARY_DIR}/${PROJECT_NAME}.o)
+add_dependencies(${PROJECT_NAME} ${PROJECT_NAME}-object)
 set_target_properties(
   ${PROJECT_NAME} PROPERTIES LIBRARY_OUTPUT_DIRECTORY
                              "${CMAKE_CURRENT_BINARY_DIR}/radix_tree")
@@ -155,6 +156,7 @@ if(HAS_VERSION_SCRIPT)
 endif()
 
 add_library(${PROJECT_NAME}-static STATIC ${CMAKE_BINARY_DIR}/${PROJECT_NAME}.o)
+add_dependencies(${PROJECT_NAME}-static ${PROJECT_NAME}-object)
 if(CMAKE_BUILD_TYPE STREQUAL Debug)
   set_target_properties(${PROJECT_NAME}-static PROPERTIES LINKER_LANGUAGE CXX)
 else()