Send EXTRA_INSTALL errors to install.log, not stderr.

We already redirected other temp-install stderr and all temp-install
stdout in this way.  Back-patch to v10, like the next commit.

Discussion: https://postgr.es/m/20181224221601.GA3227827@rfd.leadboat.com

src/Makefile.global.in

@@ -391,7 +391,7 @@ ifeq ($(MAKELEVEL),0)
 	$(MKDIR_P) '$(abs_top_builddir)'/tmp_install/log
 	$(MAKE) -C '$(top_builddir)' DESTDIR='$(abs_top_builddir)'/tmp_install install >'$(abs_top_builddir)'/tmp_install/log/install.log 2>&1
 endif
-	$(if $(EXTRA_INSTALL),for extra in $(EXTRA_INSTALL); do $(MAKE) -C '$(top_builddir)'/$$extra DESTDIR='$(abs_top_builddir)'/tmp_install install >>'$(abs_top_builddir)'/tmp_install/log/install.log || exit; done)
+	$(if $(EXTRA_INSTALL),for extra in $(EXTRA_INSTALL); do $(MAKE) -C '$(top_builddir)'/$$extra DESTDIR='$(abs_top_builddir)'/tmp_install install >>'$(abs_top_builddir)'/tmp_install/log/install.log 2>&1 || exit; done)
 endif
 endif