deps: use JCXXFLAGS for cpufeatures build

Use Julia's standard C++ flags (which include -std=c++17) instead of
bare $(CXXFLAGS). The cpufeatures public API uses std::string_view
which requires C++17.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gbaraldi

deps/cpufeatures.mk

@@ -19,7 +19,7 @@ checksum-cpufeatures: $(SRCCACHE)/cpufeatures-$(CPUFEATURES_VER).tar.gz
 $(CPUFEATURES_SRC_DIR)/build-compiled: $(CPUFEATURES_SRC_DIR)/source-extracted
 	$(MAKE) -C $(CPUFEATURES_SRC_DIR) lib \
 		CXX="$(CXX)" \
-		CXXFLAGS="$(CXXFLAGS) $(fPIC) -O2" \
+		CXXFLAGS="$(JCXXFLAGS) -O2" \
 		ARCH=$(ARCH)
 	echo 1 > $@