patch 8.2.2086: libvterm tests are only run on Linux

Problem:    Libvterm tests are only run on Linux.
Solution:   Use static libraries. (Ozaki Kiichi, closes #7419)
diff --git a/src/libvterm/Makefile b/src/libvterm/Makefile
index 852c97f..be01dd0 100644
--- a/src/libvterm/Makefile
+++ b/src/libvterm/Makefile
@@ -78,7 +78,7 @@
 	$(LIBTOOL) --mode=compile --tag=CC $(CC) $(CFLAGS) -o $@ -c $<
 
 t/harness: t/harness.lo $(LIBRARY)
-	$(LIBTOOL) --mode=link --tag=CC $(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
+	$(LIBTOOL) --mode=link --tag=CC $(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS) -static
 
 .PHONY: test
 test: $(LIBRARY) t/harness