libdl config test fails on all BSD operating systems

qtbase/config.tests/unix/libdl/libdl.pro contains

!qnx: LIBS+= -ldl

which doesn't work on any BSD operating system (Net, Open, FreeBSD) as the dl calls are part of libc and not a separate library. The test fails and the result is a build that can't load any plugins, especially platform plugins due to declaring the QT_NO_DYNAMIC_LIBRARY during compile.

The libdl.pro file should exclude freebsd, netbsd, openbsd besides qnx. The issue is urgent as we found the reasons for the failing of calling programs without failure just today and were trying to nail the cause since two weeks already. Other BSD porters will likely be hit by the same problem when trying to port 5.7 to their OS.