Add warning about using the Github Code button and check for the subm…

…odules

when running the configure script.

README.md

@@ -24,6 +24,10 @@ original CUPS `ippeveprinter` source code.
 Getting the Code
 ----------------
 
+*Do not use the ZIP file available via the Github "Code" button on the*
+*main project page, as that archive is missing the libcups and PDFio submodules*
+*submodules and will not compile.*
+
 The source code is available in semi-monthly release tarballs or via the Github
 repository.  For a release tarball, run the following commands:
 

configure.ac

@@ -1,7 +1,7 @@
 dnl
 dnl Configuration script for the IPP sample code.
 dnl
-dnl Copyright © 2014-2022 by the Printer Working Group
+dnl Copyright © 2014-2023 by the Printer Working Group
 dnl
 dnl Licensed under Apache License v2.0.  See the file "LICENSE" for more
 dnl information.
@@ -18,6 +18,16 @@ dnl We need at least autoconf 2.70 for --runstatedir...
 AC_PREREQ([2.70])
 
 
+dnl Check for submodules...
+AS_IF([test ! -d libcups -o ! -d pdfio], [
+    AS_IF([test -d .git], [
+        AC_MSG_ERROR([Run "git submodule init && git submodule update" to get the libcups and PDFio submodules.])
+    ], [
+        AC_MSG_ERROR([The ZIP archive you downloaded via the Github "Code" button is missing the libcups and PDFio submodules. Please download a dated release or use Git.])
+    ])
+])
+
+
 dnl Package name and version...
 AC_INIT([IPPSAMPLE], [YYYY.MM], [https://github.com/istopwg/ippsample/issues], [ippsample], [http://istopwg.github.io/ippsample])
 AC_CONFIG_HEADERS([config.h])