Fix Linux headless build (makefile.unix)

- Fix $(system) -> $(shell) GNU Make syntax error that broke ARCH detection
- Add obj/ and obj-test/ directory creation rules for fresh clones
- Remove duplicate -levent linkage
- Add order-only prerequisites (| obj) to pattern rules

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/makefile.unix

@@ -6,7 +6,7 @@ USE_UPNP:=0
 USE_IPV6:=1
 
 LINK:=$(CXX)
-ARCH:=$(system lscpu | head -n 1 | awk '{print $2}')
+ARCH:=$(shell lscpu | head -n 1 | awk '{print $$2}')
 
 DEFS=-DBOOST_SPIRIT_THREADSAFE
 
@@ -41,7 +41,6 @@ LIBS += \
    -l boost_chrono$(BOOST_LIB_SUFFIX) \
    -l db_cxx$(BDB_LIB_SUFFIX) \
    -l ssl \
-   -l event \
    -l crypto
 
 ifndef USE_UPNP
@@ -200,6 +199,12 @@ ifdef USE_ZMQ
     OBJS += obj/zmqpublishnotifier.o
 endif
 
+obj:
+	@mkdir -p obj
+
+obj-test:
+	@mkdir -p obj-test
+
 all: trianglesd
 
 test check: test_triangles FORCE
@@ -231,14 +236,14 @@ obj/scrypt-x86_64.o: scrypt-x86_64.S
 obj/scrypt-arm.o: scrypt-arm.S
 	$(CXX) -c $(xCXXFLAGS) -MMD -o $@ $<
 	
-obj/%.o: %.cpp
+obj/%.o: %.cpp | obj
 	$(CXX) -c $(xCXXFLAGS) -MMD -MF $(@:%.o=%.d) -o $@ $<
 	@cp $(@:%.o=%.d) $(@:%.o=%.P); \
 	  sed -e 's/#.*//' -e 's/^[^:]*: *//' -e 's/ *\\$$//' \
 	      -e '/^$$/ d' -e 's/$$/ :/' < $(@:%.o=%.d) >> $(@:%.o=%.P); \
 	  rm -f $(@:%.o=%.d)
 
-obj/%.o: %.c
+obj/%.o: %.c | obj
 	$(CXX) -c $(xCXXFLAGS) -fpermissive -MMD -MF $(@:%.o=%.d) -o $@ $<
 	@cp $(@:%.o=%.d) $(@:%.o=%.P); \
 	  sed -e 's/#.*//' -e 's/^[^:]*: *//' -e 's/ *\\$$//' \
@@ -299,7 +304,7 @@ trianglesd: $(OBJS:obj/%=obj/%)
 
 TESTOBJS := $(patsubst test/%.cpp,obj-test/%.o,$(wildcard test/*.cpp))
 
-obj-test/%.o: test/%.cpp
+obj-test/%.o: test/%.cpp | obj-test
 	$(CXX) -c $(TESTDEFS) $(xCXXFLAGS) -MMD -MF $(@:%.o=%.d) -o $@ $<
 	@cp $(@:%.o=%.d) $(@:%.o=%.P); \
 	  sed -e 's/#.*//' -e 's/^[^:]*: *//' -e 's/ *\\$$//' \