模式规则取决于另一个模式规则

Question

考虑我的makefile的以下片段：

gui_backend: $(BUILDDIR)/gui_backend

$(BUILDDIR)/gui_backend: $(BUILDDIR)/gui_backend.o $(BUILDDIR)/config.pb.o
    $(CXX) $(LDFLAGS) -o $@ $^

$(BUILDDIR)/%.o: $(SRCDIR)/%.cc $(SRCDIR)/%.h | $(BUILDDIR)
    $(CXX) $(CXXFLAGS) -c -o $@ $<

$(SRCDIR)/%.pb.cc: $(PROTODIR)/%.proto
    $(PROTOC) $(PROTOOPTIONS) --cpp_out=$(SRCDIR) $<

现在如果我打电话给make gui_backend，我明白了

make: *** No rule to make target `build/config.pb.o', needed by `build/gui_backend'.

为什么不在这里挑选$(BUILDDIR)/%.o模式？如果我通过添加规则明确说明事情

$(BUILDDIR)/config.pb.o: $(SRCDIR)/config.pb.cc $(SRCDIR)/config.pb.h | $(BUILDDIR)
    $(CXX) $(CXXFLAGS) -c -o $@ $<

它似乎工作。但是，我想让makefile尽可能简洁。

用-d运行make给我以下（complete output here）：

Considering target file `build/config.pb.o'.
       File `build/config.pb.o' does not exist.
       Looking for an implicit rule for `build/config.pb.o'.
       Trying pattern rule with stem `config.pb'.
       Trying implicit prerequisite `src/config.pb.cc'.
       Trying pattern rule with stem `config.pb'.
       Trying implicit prerequisite `build/config.pb.c'.
       Trying pattern rule with stem `config.pb'.
       Trying implicit prerequisite `build/config.pb.cc'.
       Trying pattern rule with stem `config.pb'.
       [...]
       Trying implicit prerequisite `build/SCCS/s.config.pb.o'.
       Trying pattern rule with stem `config.pb'.
       Trying implicit prerequisite `src/config.pb.cc'.
       Looking for a rule with intermediate file `src/config.pb.cc'.
        Avoiding implicit rule recursion.
        Trying pattern rule with stem `config'.
        Trying implicit prerequisite `src/proto/config.proto'.
        [...]

我正在运行GNU Make 3.81，顺便说一句。

此外，我只是注意到，如果我手动运行make src/config.pb.cc然后make build/config.pb.o，它的工作原理。

为什么这不起作用？