Fix build issues on Windows without WebKit.

We were using incorrect cmake method to set target definition to disable code
when webkit was not available. That resulted in the pqWidget_EXPORTS flag not
being set correctly, causing build issues. This fixes that problem.

Change-Id: Ie1003615f760f18c805f94e83390268bc00efaa2
Merge topic 'fix_pqWidgets_on_windows' into next

0ca4de4 Fix build issues on Windows without WebKit.