build: resolve SCHEME from $(JERBOA_HOME)/.chez/bin/scheme

ober

6d8d80b6b6be9998c4af8addbb85d5549242d2da

diff --git a/Makefile b/Makefile
index f6618c2..10eca9e 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,5 @@
-SCHEME := scheme
+JERBOA_HOME ?= $(HOME)/mine/jerboa
+SCHEME ?= $(JERBOA_HOME)/.chez/bin/scheme
 JSED_BIN := jsed
 LIBDIRS := lib