+ build-fix for ncurses-examples related to gcc8-fixes (cf: 20180526).
+ reduce use of _GNU_SOURCE for current glibc where _DEFAULT_SOURCE
combines with _XOPEN_SOURCE (Debian #900987).
+ change target configure level for _XOPEN_SOURCE to 600 to address
use of vsscanf and setenv.
+ improved configure-checks CF_SIZECHANGE and CF_STRUCT_TERMIOS from
work on dialog.