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

ober

e685a5718ad3a361aa3fed3ba81f8e1e78cfe270

diff --git a/Makefile b/Makefile
index 58aaf9f..88d9efd 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
-SCHEME = scheme
-JERBOA    = $(HOME)/mine/jerboa
+JERBOA    ?= $(HOME)/mine/jerboa
+SCHEME    ?= $(JERBOA)/.chez/bin/scheme
 JSH       = $(if $(wildcard vendor/jerboa-shell/src/jsh),vendor/jerboa-shell/src,$(HOME)/mine/jerboa-shell/src)
 COREUTILS = $(if $(wildcard $(HOME)/mine/jerboa-coreutils/lib),$(HOME)/mine/jerboa-coreutils/lib,$(HOME)/mine/jerboa-shell/vendor/jerboa-coreutils/lib)
 GHERKIN   = $(if $(wildcard vendor/gherkin-runtime),vendor/gherkin-runtime,$(HOME)/mine/gherkin/src)