Generate CFG_FILENAME_EXTRA from the version

The code takes a prefix of the MD5 hash of the version string.

Since the hash command differs across GNU and BSD platforms, we scan for
the right one in the configure script.

Closes #25007

configure

@@ -709,6 +709,20 @@ else
     probe_need CFG_GIT     git
 fi
 
+# Use `md5sum` on GNU platforms, or `md5 -q` on BSD
+probe CFG_MD5              md5
+probe CFG_MD5SUM           md5sum
+if [ -n "$CFG_MD5" ]
+then
+    CFG_HASH_COMMAND="$CFG_MD5 -q | head -c 8"
+elif [ -n "$CFG_MD5SUM" ]
+then
+    CFG_HASH_COMMAND="$CFG_MD5SUM | head -c 8"
+else
+    err 'could not find one of: md5 md5sum'
+fi
+putvar CFG_HASH_COMMAND
+
 probe CFG_CLANG            clang++
 probe CFG_CCACHE           ccache
 probe CFG_GCC              gcc