Distinguish between all and incompatible ABI changes in the check

abidiff returns 4 when the ABI has changed but not necessarily in an
incompatible way, which is something we want to allow as the new
functions may be added to the stable branch.

build/elfabi/check_all.sh

@@ -17,12 +17,21 @@ rc=0
 for l in $libraries; do
     name=$(basename $l .so)
     echo -n "Checking ${name}... "
-    if ! abidiff ${thisdir}/${name}.abi $l; then
-        echo "!!! ABI changes detected in ${name} !!!"
-        rc=1
-    else
+    abidiff ${thisdir}/${name}.abi $l
+    case $? in
+        0)
             echo 'ok'
-    fi
+            ;;
+
+        4)
+            echo "*** ABI changes detected in ${name} ***"
+            ;;
+
+        *)
+            echo "!!! INCOMPATIBLE ABI changes detected in ${name} !!!"
+            rc=1
+            ;;
+    esac
 done
 
 exit $rc