Fix indentation in Unicode configure checks
Display system name and version.
Add macro allowing to set variable and update list of required definition.

This macro will minimize error while defining variable that should
also be added to the list of required definition to allow the
both compile and run test to work as expected.

While updating the code, I also notice that there was a typo associated
with the "_NETBSD_SOURCE" definition. The variable _NETBSD_SOURCE was
set but the definition -DNETBSD_SOURCE=1 was set.

Also add an informational message indicating if XOPEN_SOURCE related
definition are set or not.
Add Large File support related flags to the list required definitions.

This is particularly useful and will ensure that the compile/run test
are also built with the appropriate definitions.

As done in the "configure.in", also set "_LARGE_FILES" to enable the
support on AIX.