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

ober

d134e881e18d64c1f1cb0dc4d4ff81daea1a113b

diff --git a/Makefile b/Makefile
index 34a042d..a149290 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
-SCHEME := scheme
 JERBOA_HOME ?= $(HOME)/mine/jerboa
+SCHEME ?= $(JERBOA_HOME)/.chez/bin/scheme
 LIBDIRS := lib:$(JERBOA_HOME)/lib
 JAWK := $(SCHEME) --libdirs "$(LIBDIRS)" --script bin/jawk.ss