use $(AR) instead of ar for building .a files, so it can be overridden in local.conf

server/Makefile

@@ -99,7 +99,7 @@ $(TARGET_GTEST): unittest_json.o unittest_nfc.o unittest_decompose.o unittest_rp
 	$(CXX) $(CPPFLAGS) $^ $(LDFLAGS) $(LDLIBS) -o $@
 
 libjson-adapter.a: $(ALL_OBJECTS)
-	ar rcs $@ $^
+	$(AR) rcs $@ $^
 
 prefix-config.cc:
 	echo '// This file is generated by make. Edit the Makefile, not this file!\n#include "prefix-config.hh" \nconst char* const html_directory = "html";\n' > $@