Explicitly specify source files for regress tools.

Since adding $(REGRESSLIBS), $? is wrong because it includes only the
changed source files.  $< seems like it'd be right however it doesn't
seem to work on some non-GNU makes, so do what works everywhere.
diff --git a/Makefile.in b/Makefile.in
index e324f65..12991cd 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -430,19 +430,19 @@
 REGRESSLIBS=libssh.a $(LIBCOMPAT)
 
 regress/modpipe$(EXEEXT): $(srcdir)/regress/modpipe.c $(REGRESSLIBS)
-	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $? \
+	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $(srcdir)/regress/modpipe.c \
 	$(LDFLAGS) -lssh -lopenbsd-compat -lssh -lopenbsd-compat $(LIBS)
 
 regress/setuid-allowed$(EXEEXT): $(srcdir)/regress/setuid-allowed.c $(REGRESSLIBS)
-	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $? \
+	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $(srcdir)/regress/setuid-allowed.c \
 	$(LDFLAGS) -lssh -lopenbsd-compat -lssh -lopenbsd-compat $(LIBS)
 
 regress/netcat$(EXEEXT): $(srcdir)/regress/netcat.c $(REGRESSLIBS)
-	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $? \
+	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $(srcdir)/regress/netcat.c \
 	$(LDFLAGS) -lssh -lopenbsd-compat -lssh -lopenbsd-compat $(LIBS)
 
 regress/check-perm$(EXEEXT): $(srcdir)/regress/check-perm.c $(REGRESSLIBS)
-	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $? \
+	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $(srcdir)/regress/check-perm.c \
 	$(LDFLAGS) -lssh -lopenbsd-compat -lssh -lopenbsd-compat $(LIBS)
 
 UNITTESTS_TEST_HELPER_OBJS=\