patch 8.0.0709: libvterm cannot use vsnprintf()

Problem:    Libvterm cannot use vsnprintf(), it does not exist in C90.
Solution:   Use vim_vsnprintf() instead.
diff --git a/src/Makefile b/src/Makefile
index 2071217..6c236ca 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -3284,7 +3284,7 @@
 Makefile:
 	@echo The name of the makefile MUST be "Makefile" (with capital M)!!!!
 
-CCCTERM = $(CCC) -Ilibvterm/include -DINLINE=""
+CCCTERM = $(CCC) -Ilibvterm/include -DINLINE="" -DVSNPRINTF=vim_vsnprintf
 objects/term_encoding.o: libvterm/src/encoding.c $(TERM_DEPS)
 	$(CCCTERM) -o $@ libvterm/src/encoding.c