PHPC-1185: Revert support for building with GSSAPI
PHPC-1185: Revert support for building with GSSAPI

libmongoc does not actually support GSSAPI (CDRIVER-2170), so we should not allow building with it. This also reverts preference for the GSS framework on macOS (PHPC-1142).

10gen-labs/mongo-php-driver-prototype,10gen-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver,mongodb/mongo-php-driver,derickr/mongo-php-driver-prototype,jmikola/mongo-php-driver,jmikola/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,derickr/mongo-php-driver,derickr/mongo-php-driver-prototype,derickr/mongo-php-driver,10gen-labs/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,derickr/mongo-php-driver,mongodb/mongo-php-driver,jmikola/mongo-php-driver-prototype,mongodb/mongo-php-driver,derickr/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,derickr/mongo-php-driver,derickr/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,derickr/mongo-php-driver,jmikola/mongo-php-driver,jmikola/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,mongodb/mongo-php-driver,jmikola/mongo-php-driver-prototype,derickr/mongo-php-driver,derickr/mongo-php-driver-prototype,mongodb/mongo-php-driver,10gen-labs/mongo-php-driver-prototype,10gen-labs/mongo-php-driver-prototype