It tries hard to recognise SDK's on different platforms. On windows MIT
Kerberos installs SDK with other things and puts path into registry.
Heimdal have separate zip archive. On linux pkg-config is tried, then
krb5-config script and finally old-style libs and headers detection.
Command line args:
* CMAKE_USE_GSSAPI - enables GSSAPI detection
* GSS_ROOT_DIR - if set, should point to the root of GSSAPI installation
(the one with include and lib directories)
