Fix issue #138: pkg-config file has incorrect Libs.private line when gssapi isn't configured
Fix issue #138: pkg-config file has incorrect Libs.private line when gssapi isn't configured

* SConstruct: When reading GSSAPI_LIBS, use empty default to not add
        "none" to Libs.private line when building without GSSAPI

Patch by: vega.james@gmail.com

jandre/serf,jandre/serf,jandre/serf,jandre/serf