Populate the link information cache before checking dependent properties.

The dependent properties check can require the link information.
Add the $<TARGET_POLICY> expression

This new expression allows checking how a policy was set when a target
was created.  That information is only recorded for a subset of policies,
so a whitelist is used.
Automatically link to the qtmain library when linking to QtCore.

When using QAxServer, ensure that the qtmain library is excluded
by reporting an error at CMake time if it is not.
Merge topic 'qt4-autolink-qtmain' into next

bdf25b7 Automatically link to the qtmain library when linking to QtCore.
6c8d8af Add the $<TARGET_POLICY> expression
1800f70 Populate the link information cache before checking dependent properties.