package/qt5/qt5enginio: switch to a depends on for SSL support

package/qt5/qt5enginio: switch to a depends on for SSL support

qt5enginio requires SSL support in qt5base. However, the SSL support
in qt5base is a bit annoying: while it can be provided by either
openssl or libressl for Qt latest, it can only be provided by
libressl for Qt 5.6.

Fabrice Fontaine initially proposed [0] a dependency on
BR2_PACKAGE_QT5BASE_OPENSSL, and a long discussion
followed. Ultimately, we found the dependency to not be nice, as it
required users to know that they need to enable some SSL
implementation to be able to enable qt5enginio.

The current solution enables BR2_PACKAGE_OPENSSL (the virtual
package), which can be either openssl or libressl. This choice was
done under the assumption that we anyway don't test Qt 5.6 in the
autobuilders. However, this is incorrect: Qt latest needs gcc >= 4.8
on host and target, and we have configurations in the autobuilders
that don't meet this requirement, and therefore build Qt 5.6, and face
a build issue due to OpenSSL being used instead of LibreSSL.

After additional thinking, this commit simply gets back to the
original solution proposed by Fabrice: a "depends on". We simply add
Config.in comments to help the user in knowing what is missing to
enable qt5enginio.

An alternate solution would have been to disallow selecting qt5enginio
when Qt 5.6 is used. But fixing the qt5enginio build is also needed
for the LTS branch, and we can't drop qt5enginio on Qt 5.6 in the LTS
branch, as that could bother users.

Fixes:

  http://autobuild.buildroot.net/results/227d4b9e2b48c5b3f2dcf0fad9eefa2816c1eb0c/

[0] https://patchwork.ozlabs.org/patch/1053883/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>