patch 8.1.0724: build for MinGW fails

Problem:    Build for MinGW fails.
Solution:   Avoid specifying dependencies in included makefile.
diff --git a/src/testdir/Makefile b/src/testdir/Makefile
index 311701a..c3ba8ab 100644
--- a/src/testdir/Makefile
+++ b/src/testdir/Makefile
@@ -33,6 +33,11 @@
 	  $(SCRIPTS_MORE2) \
 	  $(SCRIPTS_MORE4)
 
+# Explicit dependencies.
+test49.out: test49.vim
+
+test_options.res test_alot.res: opt_test.vim
+
 SCRIPTS_BENCH = bench_re_freeze.out
 
 .SUFFIXES: .in .out .res .vim