Make the project compilable with clang. (#163)

Clang generates more warning messages, such as pointing out a lack
of newline at end of file and issues with the size of a strncopy.

Moving -Wno-unused-parameter to the end of the flags avoids it being
overwritten by -Wextra.

Removing -Werror avoids compilation errors that may arise from new
compiler warnings that might come from newer versions of compilers.

src/login.c

@@ -431,7 +431,7 @@ void shell(struct passwd* pwd)
 		pos = pwd->pw_shell;
 	}
 
-	strncpy(args + 1, pos, 1024);
+	strncpy(args + 1, pos, 1023);
 	execl(pwd->pw_shell, args, NULL);
 }
 
@@ -671,4 +671,5 @@ void auth(
 	{
 		pam_diagnose(ok, buf);
 	}
-}
+}
+