the last parameter of XChangeProperty is the number of elements.. and when the element format is 32.. the element is "long" so we have 5 long elements here.

Yes this seems confusing as on mac+linux Long is either 32 or 64bits depending on the architecture, but this is how the X11 protocol is defined. Thus 5 is the correct value for the nelts here.  Not 5 or 10 depending on the architecture.

More info on the confusion https://bugs.freedesktop.org/show_bug.cgi?id=16802

build-scripts/updaterev.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+#
+# Generate a header file with the current source revision
+
+outdir=`pwd`
+cd `dirname $0`
+srcdir=..
+header=$outdir/include/SDL_revision.h
+
+rev=`sh showrev.sh 2>/dev/null`
+if [ "$rev" != "" -a "$rev" != "hg-0:baadf00d" ]; then
+    revnum=`echo $rev | sed 's,hg-\([0-9]*\).*,\1,'`
+    echo "#define SDL_REVISION \"$rev\"" >"$header.new"
+    echo "#define SDL_REVISION_NUMBER $revnum" >>"$header.new"
+    if diff $header $header.new >/dev/null 2>&1; then
+        rm "$header.new"
+    else
+        mv "$header.new" "$header"
+    fi
+fi