use predefined $(MAKE) var for SELF (#84)

Author: tamsky

Makefile

@@ -1,6 +1,6 @@
 export BUILD_HARNESS_PATH ?= $(shell 'pwd')
 export OS ?= $(shell uname -s | tr '[:upper:]' '[:lower:]')
-export SELF ?= make
+export SELF ?= $(MAKE)
 export PATH := $(BUILD_HARNESS_PATH)/vendor:$(PATH)
 export DOCKER_BUILD_FLAGS ?=