unlike speculated in 2fe36351

, this is not a workaround at all: it
causes that libraries' public link interfaces (LIBS) are exported in the
first place. unlike with staticlib, this does not export LIBS_PRIVATE,
so it wouldn't even be a particularly effective workaround for rpath
brokenness anyway.

the problem was pretty well hidden by the qt module system, which at the
level of libraries is pretty redundant with the .prl file handling,
which shows just how stupid the whole "design" is.

unlike before, we now enable explicitlib for all libraries, not just qt
modules - we enable create_prl for all of them as well, after all.

an immediate effect of this change is that it fixes linking on RaspPI:
the qtcore headers make the user code require linking libatomic, so we
must add it to our public link interface.

Task-number: QTBUG-51621
Change-Id: I5742c88694db8e8a9b79d17222dc6df2b38e5ab2
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>