Accommodate systems where paths inherit their group from the parent dir.

Thanks to Thomas Klausner <tk@giga.or.at> for reporting the problem
and helping test the solution.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Revert "Makefile: avoid using -Werror with clang for now."

Remove the clang -Werror special case after changing the way we handle
integer conversions (9ecba738e119654b7231be08232f8c484f8b1a93).

This reverts commit 3fdd538a22eae6681df3a59c116f78c237c9ecc7.

Thanks to Thomas Klausner <tk@giga.or.at> for the report.