Replace libgsasl with gsasl with GSSAPI support.

The gsasl package is a superset of libgsasl. Installing it provides the gsasl
libraries, the header files, a command line tool called gsasl to access
the library from the shell and the manual.

I wasn't able to install gsasl as its own package because it linked itself
against its own copy of libgsasl instead of the brew installed libgsasl.
This was my compromised solution to get gsasl with GSSAPI support.