export only 'host_bins' to pkg-config, rather than a random selection of tools

export only 'host_bins' to pkg-config, rather than a random selection of tools

the "export location" of the linguist tools was just bogus, and lconvert
was missing anyway. the two dbus tools and qdoc were missing, too.
generally, it seems useless to report the paths of some random tools -
instead, just report the install location of the host binaries and let
users figure out the complete paths themselves - this should be ok, as
we decided that distributors are not supposed to do tool renaming any
more.

for the binary path just use the final location, as the files won't be
used before installation anyway. this allows us removing the scary
generic prefix replace from the pc file installs.

and as a side effect this also fixes debug_and_release builds of core
and widgets by not loading various prf files prematurely and thereby
messing up the dir replacement magic.

Task-number: QTBUG-28286
Change-Id: I99de419301fc07fb923959db4bd5cab9072d1c31
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>