Add /usr/local/include for library dirs

Same as on master, but on this branch.

Released 3.2.0 2019-03-13

Lib/
* Add support for X-ORIGIN in ldap.schema's ObjectClass
* Make initialize() pass extra keyword arguments to LDAPObject
* ldap.controls.sss: use str instead of basestring on Python 3
* Provide ldap._trace_* atributes in non-debug mode

Doc/
* Fix ReST syntax for links to set_option and get_option

Tests/
* Use intersphinx to link to Python documentation
* Correct type of some attribute values to bytes
* Use system-specific ENOTCONN value

Infrastructure:
* Add testing and document support for Python 3.7
* Add Python 3.8-dev to Tox and CI configuration
* Add Doc/requirements.txt for building on Read the Docs

Bump version to 3.1.0

Add changelog and bump version to 3.0.0

Bump version to 3.0.0b4 and update CHANGES

python-ldap#158

Bump version to 3.0.0b3

Update CHANGES for 3.0.0b2

Accept more error messages in test_tls_ext_noca

OpenSSL 1.0, 1.1, and NSS return different error messages for untrusted
certificate and missing CA.

python-ldap#92
Closes: python-ldap#87
Signed-off-by: Christian Heimes <cheimes@redhat.com>

use single quotes

prepare release 2.5.1