fix(library/Makefile.common): avoid error message when .d files do not exist

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

library/Makefile.common

@@ -20,7 +20,7 @@ all: $(OLEAN_FILES) $(DEP_FILES)
 	@rm -f $@.tmp.1
 	@rm -f $@.tmp.2
 
-include $(LEAN_FILES:.lean=.d)
+-include $(LEAN_FILES:.lean=.d)
 
 .PHONY: all clean
 

src/CMakeLists.txt

@@ -328,4 +328,9 @@ endif()
 add_custom_target(clean-olean
    WORKING_DIRECTORY ${LEAN_SOURCE_DIR}/../library
    COMMAND find . -name '*.olean' -exec rm -f '{}' '\;'
-)
+   )
+
+add_custom_target(clean-d
+  WORKING_DIRECTORY ${LEAN_SOURCE_DIR}/../library
+  COMMAND find . -name '*.d' -exec rm -f '{}' '\;'
+  )