Make dependency on unistd.h specific to HAVE_CURL instead in the whole file
Make dependency on unistd.h specific to HAVE_CURL instead in the whole file

- introduced in COUCHDB-1338
- allows building cleanly on Windows without cURL

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb