Some GSS-API functions like gss_import_name(), gss_release_buffer()
used in plugin/auth_gssapi and libmariadb/plugins/auth are marked
as deprecated in MacOS starting from  version 10.14+. It results in
extra warnings output on server building.

To eliminate extra warnings the flag '-Wno-deprecated-declarations'
has been added to compiler invocation string for those source
files that invoke deprecated GSS-API functions.