Merge openldap-2.5.6.

Update for building OpenLDAP 2.4.39.

Default to -Wno-sign-compare -Wno-pointer-sign for clang.
Push -Wno-array-bounds down to the cases that depend on it.
Selectively disable warnings for 3rd party software or non-trivial
issues to be r

Default to -Wno-sign-compare -Wno-pointer-sign for clang.
Push -Wno-array-bounds down to the cases that depend on it.
Selectively disable warnings for 3rd party software or non-trivial
issues to be reviewed later to get clang -Werror to build most of the
tree.

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Add USE_FORT?=yes.

NetBSD reachover build framework for OpenLDAP.
Inspired by framework kindly donated by Wasabi Systems.