Clean up example makefile to properly support externally supported paths. (This is a tiny fix to PR 193)

examples/make_example/Makefile

@@ -1,8 +1,8 @@
 CC ?= gcc
-BUILD_DIR ?= ./build
-SRC_DIR ?= ./src
-TEST_DIR ?= ./test
-TEST_BUILD_DIR ?= ${BUILD_DIR}/test
+export BUILD_DIR ?= ./build
+export SRC_DIR ?= ./src
+export TEST_DIR ?= ./test
+export TEST_BUILD_DIR ?= ${BUILD_DIR}/test
 TEST_MAKEFILE = ${TEST_BUILD_DIR}/MakefileTestSupport
 OBJ ?= ${BUILD_DIR}/obj
 OBJ_DIR = ${OBJ}