qmake: Add support for QMAKE_{PRE,POST}_LINK in the Xcode generator

Change-Id: I038cf0aebb74d7ecfe6cb3ed868287042342eb7e
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
iOS: Wrap user's main by renaming symbol and relying on weak linking

This approach is similar to the earlier apprach of defining main=qt_main
when building the user's sources, but uses the linker to rename the
symbol instead, which is less fragile than using the preprocessor.

To keep the hybrid usecase unaffected by our wrapper logic we declare
both our main wrapper and a fallback qt_main as weak symbols, which
ensures that when the user's application links in our plugin the
real main/qt_main provided by the user is preferred over our weak
symbols.

Change-Id: Ic76f3ba8932430c4b13a1d3a40b8ed2322fe5eea
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>