FWIW I have confirmed that every packages in sid/i386 shipping
/usr/include/GL  does so with the correct permissions.
And of course setting umask does not break dpkg's ability to generate
0755 directories. (Tested with umask 0077).
The only thing I could think of is a package that generates
/usr/include/GL in its postinst without specifying the mode.
`What a good friend you are to him, Dr. Maturin. His other friends are
so grateful to you.'
`I sew his ears on from time to time, sure'