FindCUDA: Always list custom command outputs in their targets

CMake's intended interface for linking to explicit object files (marked
with EXTERNAL_OBJECT) is that only those listed as target sources should
be linked.  Drop FindCUDA's attempt to hide the .obj files from VS IDE
project files, which depends on VS-version-specific behavior of linking
custom command outputs that happen to be named "*.obj".  CMake puts
external object files in a dedicated source group anyway.
Comment the test to explain why the elements are there.
Merge topic 'fix-genex-preprocessing-incomplete-test' into next

a31f1af Comment the test to explain why the elements are there.
genex: Test preprocessing incomplete expressions.
Merge topic 'fix-genex-preprocessing-incomplete-test' into next

2aafacc genex: Test preprocessing incomplete expressions.
Merge topic 'FindCUDA-list-obj-files' into next

ef27fa6 FindCUDA: Always list custom command outputs in their targets