config.mak is no longer included multiple times; the guards are pointless.
cat > config.mak <<EOF
# Automatically generated by configure - do not modify!
-ifndef LIBAV_CONFIG_MAK
-LIBAV_CONFIG_MAK=1
LIBAV_CONFIGURATION=$LIBAV_CONFIGURATION
prefix=$prefix
LIBDIR=\$(DESTDIR)$libdir
EOF
echo "#endif /* LIBAV_CONFIG_H */" >> $TMPH
-echo "endif # LIBAV_CONFIG_MAK" >> config.mak
# Do not overwrite an unchanged config.h to avoid superfluous rebuilds.
cp_if_changed $TMPH config.h