Link libcrash conditionally.

This enables building metrics for the host thus enabling using metrics
in the current update_engine without breaking the host dev flow.

BUG=4852
TEST=emerged with and without USE=-crash

Review URL: http://codereview.chromium.org/3032004

metrics/Makefile

@@ -18,6 +18,8 @@ SHAREDLIB = libmetrics.so
 LIB_TEST = metrics_library_test
 COUNTER_TEST = counter_test
 
+LCRASH ?= -lcrash
+
 TESTCOUNTER_OBJS = \
 	counter.o \
 	counter_test.o
@@ -53,7 +55,7 @@ $(COUNTER_TEST): $(TESTCOUNTER_OBJS)
 	$(CXX) -o $@ $^ $(TESTCOUNTER_LIBS)
 
 $(DAEMON): $(DAEMON_OBJS) $(SHAREDLIB)
-	$(CXX) -o $@ $^ $(DAEMON_LDFLAGS) -lcrash
+	$(CXX) -o $@ $^ $(DAEMON_LDFLAGS) $(LCRASH)
 
 $(DAEMON_TEST): $(TESTDAEMON_OBJS)
 	$(CXX) -o $@ $^ $(DAEMON_LDFLAGS) $(TESTDAEMON_LIBS)