Build instructions for Qt 5.15 updated for 5.15.7

The Qt 5.15 build tutorial has been updated to reflect the new release, 5,15.7. This release is the latest version in Qt 5.15 branch, 5.15.7, that was released in 28.10.2022.

The release is a patch release that does not introduce any new features. It includes bug fixes only, with 12 important bug fixes, many security fixes in qtwebengine and many other bug fixes in other modules. Sources for included 3rd party components has been updated, SQLite to v3.36.0, libjpeg-turbo to version 2.1.1, libwebp to version 1.2.1 and PCRE2 to 10.38.

For in-depth details see the release notes.