This change allows using custom flags for ar. For instance, it is
now possible to create a thin archive by setting QMAKE_LIB += -T.
This uses and extends commit d92c25b1b4
which served a similar purpose for the linker.
Change-Id: Ie1d6a0b957dc4809957726de00911c8d91647fab
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>