--- /home/vagrant/ruby-2.5.0/template/configure-ext.mk.tmpl	2017-04-23 16:19:22.000000000 +0000
+++ ./template/configure-ext.mk.tmpl	2018-02-17 23:31:02.304445580 +0000
@@ -24,7 +24,7 @@
 MINIRUBY = <%=miniruby%>
 SCRIPT_ARGS = <%=script_args%>
 EXTMK_ARGS = $(SCRIPT_ARGS) --gnumake=$(gnumake) --extflags="$(EXTLDFLAGS)" \
-	   --make-flags='MINIRUBY=$(MINIRUBY)'
+	   --make-flags="MINIRUBY='$(MINIRUBY)'"
 
 all: exts gems
 exts: