meson: handle missing ldap dependency

Adjust the meson.build so that ldap can be an optional dependency.
This allows out-of-docker builds for environments which might not have
the LDAP libraries installed.  In the future this could also be
leveraged by a meson.option that would disable LDAP support.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I214b7e69580bd11432f88a85fa760a5e07f7fad2
3 files changed