use join-with that works in GNU Make 4.3

no substitution was occurring for the HPP_FILES list, causing the
merge-headers to panic and fail because the command arguments were not
split properly when executing the command from DependencyGraph.new

```
thread 'main' panicked at 'clang++-11 ["-MT", "alloc.hpp api.hpp bytes.hpp comment.hpp decoded_input.hpp decoder.hpp diagnostic.hpp loc.hpp magic_comment.hpp messages.hpp nodes.hpp parser_options.hpp parser_result.hpp shared_byte_list.hpp source_line.hpp string.hpp token_rewriter.hpp token.hpp token_ids.hpp", "-MM", "alloc.hpp api.hpp bytes.hpp comment.hpp decoded_input.hpp decoder.hpp diagnostic.hpp loc.hpp magic_comment.hpp messages.hpp nodes.hpp parser_options.hpp parser_result.hpp shared_byte_list.hpp source_line.hpp string.hpp token_rewriter.hpp token.hpp token_ids.hpp"] returns incompatible output:
', src/dependency_graph.rs:28:9
```

Makefile

@@ -22,10 +22,10 @@ SOURCES = \
 	token_rewriter \
 	token
 
-CPP_FILES = $(foreach source,$(SOURCES),$(source).cpp)
-HPP_FILES = $(foreach source,$(SOURCES),$(source).hpp)
+CPP_FILES = $(addsuffix .cpp,$(SOURCES))
+HPP_FILES = $(addsuffix .hpp,$(SOURCES))
 HPP_FILES += token_ids.hpp
-O_FILES = $(foreach source,$(SOURCES),$(source).$(O))
+O_FILES = $(addsuffix .$(O),$(SOURCES))
 STATIC_LIB = libruby_parser_cpp.$(A)
 
 # Codegen

codegen/build.mk

@@ -59,9 +59,8 @@ codegen/merge-headers:
 	chmod +x codegen/merge-headers
 CLEAN += codegen/merge-headers
 
-# A literal space.
-space :=
-space +=
+empty :=
+space := $(empty) $(empty)
 
 # Joins elements of the list in arg 2 with the given separator.
 #   1. Element separator.