YJIT, ZJIT: warning: overriding recipe for target on combo build

[k0kubun]

When I build Ruby with --enable-yjit --enable-zjit, it gives warnings:

../zjit/zjit.mk:29: warning: overriding recipe for target '/home/k0kubun/src/github.com/ruby/ruby/.ruby-release/'
../yjit/yjit.mk:25: warning: ignoring old recipe for target '/home/k0kubun/src/github.com/ruby/ruby/.ruby-release/'

which seems to come from:

BUILD_YJIT_LIBS = $(TOP_BUILD_DIR)/$(YJIT_LIBS)

# YJIT_SUPPORT=yes when `configure` gets `--enable-yjit`
ifeq ($(YJIT_SUPPORT),yes)
yjit-libs: $(BUILD_YJIT_LIBS)
$(BUILD_YJIT_LIBS): $(YJIT_SRC_FILES)
	$(ECHO) 'building Rust YJIT (release mode)'
...

and

BUILD_ZJIT_LIBS = $(TOP_BUILD_DIR)/$(ZJIT_LIBS)

# ZJIT_SUPPORT=yes when `configure` gets `--enable-zjit`
$(BUILD_ZJIT_LIBS): $(ZJIT_SRC_FILES)
	$(ECHO) 'building Rust ZJIT (release mode)'
...

It seems to be because the combo build resets $(YJIT_LIBS) and $(ZJIT_LIBS), so $(BUILD_YJIT_LIBS) becomes equal to $(BUILD_ZJIT_LIBS).

I'm not sure if it's harmful, but it'd be nice to suppress the warnings.