Set _GNU_SOURCE globally

Add gentoo distrubutions Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>

Remove bash-isms from configure macros

Allow Debian/Ubuntu build to pass --install-layout=deb to setup.py

Use SO_PEERCRED on the PAM socket This is the second attempt to let the PAM client and the PAM responder exchange their credentials, i.e. uid, gid and pid. Because this approach does not require any message interchange between the client and the server the protocol version number is not changed. On the client side the connection is terminated it the responder is not run by root. On the server side the effective uid and gid and the pid of the client are available for future use. The following additional changes are made by this patch: - the checks of the ownership and the permissions on the PAM sockets are enhanced - internal error codes are introduced on the client side to generate more specific log messages if an error occurs

Revert "Add better checks on PAM socket" This reverts commit 5a88e963744e5da453e88b5c36499f04712df097.

Add better checks on PAM socket - check if the public socket belongs to root and has 0666 permissions - use a SCM_CREDENTIALS message if available

Rename server/ directory to src/ Also update BUILD.txt