Historically, the mongodb-sasl configure option has accepted "auto", "no", and a path to be used for searching for Cyrus SASL headers. We should instead rely on pkg-config (i.e. PKG_CHECK_MODULES) for finding the library with a possible fallback to PHP_CHECK_LIBRARY for consistency with CheckSSL.m4. Do not accept a path value to search for Cyrus SASL header files. For consistency with mongodb-ssl, we can accept a "cyrus" value for the option, which will allow users to specify that Cyrsu SASL must be found or else an error should be raised (unlike "auto").

Previously, this issue suggested also accepting "gssapi" for mongodb-sasl; however, we later realized that libmongoc never fully implemented support for GSSAPI (see: CDRIVER-2170).