4.9.2: move QT_NO_DEBUG conditional out of mkspecs

Binary packages for Debian 10 (amd64, s390x and hurd), Fedora 33 (amd64), Arch Linux (amd64) and FreeBSD 12.2 (i386) provided.

Many compiler warnings, static analyzer warnings and miscellaneous issues have been fixed. For complete list of changes done since 4.9.1 visit this link.

This release is most likely the last of the 4.9 series, for a list of things to be done before the next release which will be 4.10 visit this link. Note that the tasks list is not complete and new tasks may be added.

⚠️There is no guarantee that the provided packages will continue to work for the day after tomorrow as some distributions purposly choose to move on at pace that often breaks compatibility in one way or other. Future releases will not include packages for Fedora and Arch Linux, you will have to build these yourself via the provided package files. For instructions on how to do that visit this link.