patch 8.2.2432: libvterm tests are executed even when libtool doesn't work

Problem:    Libvterm tests are executed even when libtool doesn't work.
Solution:   Only run libvterm tests if /usr/bin/gcc exists.
diff --git a/src/Makefile b/src/Makefile
index c30af8b..4e32814 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2294,9 +2294,12 @@
 
 # Run the libvterm tests.
 # This works only on GNU make, not on BSD make.
+# Libtool requires "gcc".
 test_libvterm:
 	@if $(MAKE) --version 2>/dev/null | grep -qs "GNU Make"; then \
-		cd libvterm; $(MAKE) -f Makefile test CC="$(CC)" CFLAGS="$(CFLAGS)" LDFLAGS="$(LDFLAGS)"; \
+	  if test -x "/usr/bin/gcc"; then \
+	    cd libvterm; $(MAKE) -f Makefile test CC="$(CC)" CFLAGS="$(CFLAGS)" LDFLAGS="$(LDFLAGS)"; \
+	  fi \
 	fi
 
 # Run individual OLD style test.