Add support for PRIVATE header sets, analogous to the existing
INTERFACE header sets support. A key difference between the two is
the way they handle build and usage requirements. INTERFACE
header sets rely on the original target's INTERFACE, whereas PRIVATE
header sets copy across various properties from the original target.
Generated files need special handling for PRIVATE header sets.
Add a dependency on the original target to the verification target to
ensure such generated files will exist.
Fixes: #23448
The CMAKE_VERIFY_INTERFACE_HEADER_SETS variable is intended to
be under the control of the user. It doesn't discriminate between
header sets defined in the main project and those defined by
dependencies brought into the build directly via FetchContent.
Developers will usually only be interested in verifying the main project's
header sets, not those from dependencies.
Make the variable effectively only enable header set verification of the
main project by turning it off during FetchContent_MakeAvailable() calls.
The user still has variables like CMAKE_PROJECT_INCLUDE and
CMAKE_PROJECT_<projectName>_INCLUDE available to them if they
want to enable verification of all or specific dependencies respectively.
Fixes: #23808
Craig Scott