Expand the 'existing target guard' in generated CMake files.

Expand the 'existing target guard' in generated CMake files.

This way the target will be created and have its properties
populated only one time.

I tried wrapping the whole file in an 'include guard', but that
broke the unit test in tests/auto/cmake/pass1 (and
the qt5_use_module function), because the function causes the
variables in the Config file to not exist outside of the
scope (eg for include directories), and yet, Qt5${Module}_FOUND is
still true even when the find_package was previously called in a
function, so it is not found and processed again.

The change in Qt5CoreConfigExtras.cmake does not need to be guarded
as it is only ever included from Qt5CoreConfig.

Change-Id: Iaa016563db5eb61294360ac9e003c9c923393d8c
Reviewed-by: Brad King <brad.king@kitware.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>