Add option to configure curl with GSSAPI support.

Tested locally, and had no issues on OS 10.7.5.

<pre>
$ brew install curl --with-gssapi --with-darwinssl
==> Downloading http://curl.haxx.se/download/curl-7.32.0.tar.gz
Already downloaded: /Library/Caches/Homebrew/curl-7.32.0.tar.gz
==> ./configure --prefix=/usr/local/Cellar/curl/7.32.0 --with-darwinssl --with-gssapi
==> make install
==> Caveats
[..snip caveats..]
==> Summary
/usr/local/Cellar/curl/7.32.0: 74 files, 2.0M, built in 44 seconds


$ /usr/local/Cellar/curl/7.32.0/bin/curl -V
curl 7.32.0 (x86_64-apple-darwin11.4.2) libcurl/7.32.0 SecureTransport zlib/1.2.5
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtsp smtp smtps telnet tftp
Features: GSS-Negotiate IPv6 Largefile NTLM NTLM_WB SSL libz
</pre>

Compiled successfully with GSS-Negotiate available.