4.9.0-rc19: remove redundant __has_extension definition in qglobal header

Binary packages for Debian 10 (i386 and armhf) and FreeBSD (i386) provided, highlights:

• POSIX signals support is required
• libpng and Sqlite are required
• Zstandard atleast v1.1.3 is required
• OpenSSL atleast v0.9.8 is required
• Linux atleast v2.6.27 is required
• glibc atleast v2.9 is required
• Linux specific CLOCK_MONOTONIC_COARSE is used in timers if possible
• updated Unicode data tables to v13.0
• updated CLDR data tables to v36.1
• updated URL TLDs data tables
• improved Qt4 compatibility for projects using CMake
• improved thread-safe resolv library functions check during build
• fixed find_package() query with version requirement by projects using CMake, affects other projects
• fixed build without X11 session management support for some cases
• fixed build with PostgreSQL support for some cases
• fixed build on recent versions of FreeBSD
• fixed QWidget::setWindowTitle() thread-safety issue
• fixed headers optimization via unifdef
• fixed regression since 28d63e9
• fixed regression since fa34c08
• fixed regression since 6323734
• fixed regression since 0199f10 and cc7809a
• fixed loading of plugins and libraries with a relative file name (upstream qt/qtbase@418890e)
• dropped support for CPack
• dropped support for static plugins
• dropped MNG format support
• dropped obscure (24-bit) QImage formats support
• lots of cleanups, optimizations and fixes