check_headers: fix the script (a partial match is not enough).

src/check_headers

@@ -9,7 +9,8 @@ regexp="$(cat ../config.h.in | \
 	sed -n -e 's/^#undef \([A-Z0-9_]*\)$/\1/p' | \
 	grep -v 'WORDS_BIGENDIAN' | \
 	xargs | \
-	sed -e 's/ /\\\|/g')"
+    sed -e 's/ /\\\| /g' | sed -e 's/\\|/\$\\|/g')"
+regexp=" $regexp\$"
 
 echo Looking for private defines in public headers...
 ! grep -- "$regexp" "$@" || exit $?