Some platforms (e.g. FreeBSD) do not have libdl, but dlopen and related
functions are part of libc. So first check for dlopen in libc, and only
if that fails, look for it in libdl.
Task-number: QTBUG-52951
Change-Id: I65a8ed18fce157da32f4e1ffeba30d7513385a8f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
