patch 9.0.1628: syntax tests fail on FreeBSD

Problem:    Syntax tests fail on FreeBSD.
Solution:   Pass the Vim executable path with VIMPROG. (Ken Takata,
            closes #12535)  Adjust the paths.
diff --git a/Makefile b/Makefile
index 6e68093..f70fb85 100644
--- a/Makefile
+++ b/Makefile
@@ -66,7 +66,7 @@
 syntaxtest:
 	cd runtime/syntax && \
 		$(MAKE) clean && \
-		$(MAKE) test VIM="$(VIM_FOR_SYNTAXTEST)"
+		$(MAKE) test VIMPROG="$(VIM_FOR_SYNTAXTEST)"
 
 
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