Cmake: Build with GSSAPI (MIT or Heimdal)

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)