py-gssapi: Mention GSSAPI source in longdesc

This change updates the port's long_description to specify which GSSAPI
implementation it is using.

py-gssapi needs a GSSAPI/Kerberos library to build against.  On Mac OS X
10.8 and earlier, py-gssapi uses the `kerberos5` port.  On Mac OS X 10.9
and later, py-gssapi uses the OS-provided GSS Framework.

The specific GSSAPI implementation is useful to know, because a Kerberos
credential that is kinit-ed from MIT Kerberos might not be visible to a
program that is using macOS' GSS Framework.

python/py-gssapi/Portfile

@@ -35,6 +35,10 @@ if {${name} ne ${subport}} {
     # Uses GSS.framework on macOS >= 10.9
     if {${os.platform} eq "darwin" && ${os.major} < 13} {
         depends_lib-append  port:kerberos5
+        long_description-append  Uses GSSAPI/Kerberos from MacPorts' \
+        kerberos5 package.
+    } else {
+        long_description-append  Uses macOS' built-in GSSAPI/Kerberos.
     }
 
     livecheck.type          none