patch 8.0.0985: libvterm has its own idea of character width

Problem:    Libvterm has its own idea of character width.
Solution:   Use the Vim functions for character width and composing to avoid a
            mismatch. (idea by Yasuhiro Matsumoto)
diff --git a/src/Makefile b/src/Makefile
index 1f252f0..e4da3b4 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -3296,7 +3296,11 @@
 Makefile:
 	@echo The name of the makefile MUST be "Makefile" (with capital M)!!!!
 
-CCCTERM = $(CCC) -Ilibvterm/include -DINLINE="" -DVSNPRINTF=vim_vsnprintf
+CCCTERM = $(CCC) -Ilibvterm/include -DINLINE="" \
+	  -DVSNPRINTF=vim_vsnprintf \
+	  -DIS_COMBINING_FUNCTION=utf_iscomposing_uint \
+	  -DWCWIDTH_FUNCTION=utf_uint2cells
+
 objects/term_encoding.o: libvterm/src/encoding.c $(TERM_DEPS)
 	$(CCCTERM) -o $@ libvterm/src/encoding.c