verify-private-header-sets

• When VERIFY_INTERFACE_HEADER_SETS is set to true on an executable target, that target's interface file sets are verified regardless of its ENABLE_EXPORTS property. See policy CMP0209.

• The CMAKE_VERIFY_PRIVATE_HEADER_SETS variable and corresponding VERIFY_PRIVATE_HEADER_SETS target property were added to enable build rules that verify all headers in private file sets can be used on their own.

• The PRIVATE_HEADER_SETS_TO_VERIFY target property was added to customize which private file sets to verify when the target's VERIFY_PRIVATE_HEADER_SETS property is true.