Add update make target, update submodule and fix config

I have just wasted a couple of hours because the zipflow commit
used here was not the one I used in the independently checked
out tree, so I thought we had had this commit, but had not:

commit c4622bfba95577a24e34112934afffeb3cc2f822
Author:     Mark Adler <madler@alumni.caltech.edu>
AuthorDate: Tue Jan 3 04:08:51 2023 -0800
Commit:     Mark Adler <madler@alumni.caltech.edu>
CommitDate: Tue Jan 3 04:12:37 2023 -0800

    Fix bugs in zip_data() for large lengths.

.gitmodules

-[submodule "foreign/zipflow"]
+[submodule "src/foreign/zipflow"]
 	path = src/foreign/zipflow
 	url = https://github.com/madler/zipflow.git

Makefile.am

@@ -12,3 +12,7 @@ noinst_DATA = README.md INSTALL.md
 BUILT_SOURCES = $(noinst_DATA)
 
 DISTCLEANFILES = $(noinst_DATA)
+
+update:
+	git submodule update --remote src/foreign/zipflow
+	git commit -am 'Update submodule'

zipflow @ 2bef2123

-Subproject commit 95ae59fc13f684a19dc2f4875eaba4410564c2a6
+Subproject commit 2bef2123ebe519c17b18d2d0c3c71065088de952