Don't behave differently during try_compiles.

It doesn't make sense to behave differently.

Also update the try_compile tests to actually do tests. Previously
they were not run or checked properly.
Merge topic 'IMPORTED-target-SYSTEM-includes' into next

b215bce Don't behave differently during try_compiles.
Always consider includes from IMPORTED targets to be SYSTEM.

Introduce a target property to control this behavior variable
to set the default value for the target property.

This does not affect try_compile runs.
Merge topic 'IMPORTED-target-SYSTEM-includes' into next

a63fcbc Always consider includes from IMPORTED targets to be SYSTEM.