Remove _DARWIN_C_SOURCE definition for bundled sources on macOS
Remove _DARWIN_C_SOURCE definition for bundled sources on macOS

Reverts a change introduced in ab44b0cbeb52361db568e8a506a78d038544d3d5. This was previously in mongodb/mongo-c-driver#920 but removed before merging in mongodb/mongo-c-driver@9d2d8b157ce8ffc53db994b827c2e28267f079e9.

mongodb-labs/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,mongodb/mongo-php-driver,jmikola/mongo-php-driver,jmikola/mongo-php-driver,mongodb/mongo-php-driver,jmikola/mongo-php-driver,mongodb/mongo-php-driver,mongodb/mongo-php-driver,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype