fedabipkgdiff: Enable testing without proper Koji installation

diff --git a/CONTRIBUTING b/CONTRIBUTING
index 24a483b2..5ab7bcce 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -174,6 +174,26 @@  make sure to add your new test input files to the
 tests/data/Makefile.am file, in the EXTRA_DIST variable.  Look at how
 things are organized in that file, and please do things similarly.
 
+fedabipkgdiff testing
+---------------------
+
+This depends on the Python koji and a few support modules.  If these are
+not available, 'configure' disables fedabipkgdiff and its 'make check'
+testing.
+
+Debian and Ubuntu, for example, don't provide packages for the Python
+koji module.  If you'd like to run fedabipkgdiff testing without manually
+doing a proper Koji installation, you may simply download Koji sources
+(<https://pagure.io/koji>, releases, or Git clone) and point to these via
+the Python search path.  For example, tested 2021-12-11:
+
+  wget https://pagure.io/koji/archive/koji-1.27.0/koji-koji-1.27.0.tar.gz
+  tar -xzf koji-koji-1.27.0.tar.gz
+  export PYTHONPATH="$PWD"/koji-koji-1.27.0
+
+A libabigail 'configure' run then enables fedabipkgdiff and its
+'make check' testing.
+
 Coding language and style
 ==========================
 
diff --git a/configure.ac b/configure.ac
index 40f57ed9..29130175 100644
--- a/configure.ac
+++ b/configure.ac
@@ -588,7 +588,11 @@  if test x$CHECK_DEPS_FOR_FEDABIPKGDIFF = xyes; then
     AC_MSG_CHECKING([if koji client is recent enough])
     $PYTHON 2>&AS_MESSAGE_LOG_FD -c "
 import koji
-koji.read_config('koji')"
+try:
+    koji.read_config('koji')
+except koji.ConfigurationError:
+    # See 'tools/fedabipkgdiff'.
+    pass"
     if test $? -eq 0; then
       koji_version_check_ok=yes
     else
diff --git a/tools/fedabipkgdiff b/tools/fedabipkgdiff
index 89cbd15c..be4182b2 100755
--- a/tools/fedabipkgdiff
+++ b/tools/fedabipkgdiff
@@ -46,9 +46,20 @@  import koji
 # something wrong.
 
 
-koji_config = koji.read_config('koji')
-DEFAULT_KOJI_SERVER = koji_config['server']
-DEFAULT_KOJI_TOPURL = koji_config['topurl']
+# First, try proper Koji initialization.
+try:
+    koji_config = koji.read_config('koji')
+    DEFAULT_KOJI_SERVER = koji_config['server']
+    DEFAULT_KOJI_TOPURL = koji_config['topurl']
+except koji.ConfigurationError:
+    # ..., but if that fails because of a rather strict interpretation where
+    # 'read_config' looks for configuration files, just use dummy values.
+    # These fail upon use unless overridden, which for libabigail test suite
+    # usage they always are (all relevant artifacts are shipped in the
+    # libabigail distribution).
+    DEFAULT_KOJI_SERVER = 'dummy_DEFAULT_KOJI_SERVER'
+    DEFAULT_KOJI_TOPURL = 'dummy_DEFAULT_KOJI_TOPURL'
+
 
 # The working directory where to hold all data including downloaded RPM
 # packages Currently, it's not configurable and hardcode here. In the future