On Windows, if Qt is built with Vulkan support but the user's machine
does not have Vulkan, it should still be possible to configure and
build an application (if the application does not use Vulkan of
course).
When Qt is built with qmake, the special windows_vulkan_sdk.prf file
makes sure not to export build time Vulkan include headers into the
generated .pri files. The same file also tries to find the include
headers via an environment variable. If it isn't set, it just adds a
bogus "/include" include path, which doesn't fail a user's application
build.
This wasn't the case for an application built with CMake, because the
exported Vulkan_nolink target uncodinitionally referenced Vulkan's
target properties. Which means that if the Vulkan package was not
found, the application failed to configure.
To mimic qmake's behavior, make sure to query the target properties
only if the Vulkan target exists, via the TARGET_EXISTS generator
expression.
Apply the same logic to all _nolink targets. This might not be
entirely correct in all cases, but we can revise this behavior later
after more feedback. At the very least it allows building non-Vulkan
based applications.
Task-number: QTBUG-85240
Change-Id: Iffbb03a84e8637ed54d0811433e66fe6de43d71f
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
7b9904849f