Makefile: use which instead of command (open-telemetry#672)

Makefile

@@ -1,7 +1,7 @@
 HTMLTEST_DIR=tmp
 HTMLTEST?=htmltest # Specify as make arg if different
 # Use $(HTMLTEST) in PATH, if available; otherwise, we'll get a copy
-ifeq (, $(shell command -v $(HTMLTEST)))
+ifeq (, $(shell which $(HTMLTEST)))
 PRE_CHECK_LINKS=get-link-checker
 override HTMLTEST=$(HTMLTEST_DIR)/bin/htmltest
 endif