build: Pass BSDmakefile args to gmake

Minor convenience for platforms that doesn't have gmake installed
but prefer the habit of writing make instead of gmake.

test needs to live in .PHONY to get passed on to gmake.

BSDmakefile

@@ -1,2 +1,8 @@
-all:
-	@echo "I need GNU make. Please run \`gmake\` instead."
+all: .DEFAULT
+.DEFAULT:
+	@which gmake > /dev/null 2>&1 ||\
+		(echo "GMake is required for io.js to build.\
+			Install and try again" && exit 1)
+	@gmake ${.MAKEFLAGS} ${.TARGETS}
+
+.PHONY: test