Update mkrel for SPHINX_ARGS

Update mkrel so that it will explicitly set SPHINX_ARGS=-W to make
warnings fatal and so that it will work in an unconfigured tree.

(cherry picked from commit 6fbc0e5)

ticket: 7497
version_fixed: 1.11
status: resolved

src/util/mkrel

@@ -186,9 +186,9 @@ rm -rf $reldir/.git || true
 if test $dodoc = t; then
 	echo "Building doc..."
 	(cd $reldir/src/doc && make -f Makefile.in \
-	    top_srcdir=.. srcdir=. PYTHON=python html pdf)
+	    top_srcdir=.. srcdir=. SPHINX_ARGS=-W PYTHON=python html pdf)
 	(cd $reldir/src/doc && make -f Makefile.in \
-	    top_srcdir=.. srcdir=. PYTHON=python clean)
+	    top_srcdir=.. srcdir=. SPHINX_ARGS=-W PYTHON=python clean)
 fi
 
 echo "Generating tarfiles..."