Add the INTERFACE_LIBRARY target type.

This target type only contains INTERFACE_* properties, so it can be
used as a structural node. The target-specific commands enforce
that they may only be used with the INTERFACE keyword when used
with INTERFACE_LIBRARY targets. The old-style target properties
matching LINK_INTERFACE_LIBRARIES_<CONFIG> are always ignored for
this target type.

The name of the INTERFACE_LIBRARY must match a validity generator
expression. The validity is similar to that of an ALIAS target,
but with the additional restriction that it may not contain
double colons. Double colons will carry the meaning of IMPORTED
or ALIAS targets in CMake 2.8.13.

An ALIAS target may be created for an INTERFACE library.

At this point it can not be exported and does not appear in the
buildsystem and project files are not created for them. That may
be added as a feature in a later commit.

The generators need some changes to handle the INTERFACE_LIBRARY
targets returned by cmComputeLinkInterface::GetItems. The Ninja
generator does not use that API, so it doesn't require changes
related to that.
export: Add policy CMP0024 to disallow include() of export files

Currently, export() is executed at configure-time.

One problem with this is that certain exported properties like
the link interface may not be complete at the point the export() is
encountered leading to an incorrect or incomplete exported
representation. Additionally, the generated IMPORTED_LOCATION
property may even be incorrect if commands following the export()
have an effect on it.

Another problem is that it requires the C++ implementation of cmake
to be capable of computing the exported information at configure time.
This is a limitation on the cleanup and maintenance of the code. At
some point in the future, this limitation will be dropped and more
implementation will be moved from cmTarget to cmGeneratorTarget.
export: Update approximation of version 2.8.13 to be over 2.8.12
export: Add support for INTERFACE_LIBRARY targets
install: Teach EXPORT option to handle INTERFACE_LIBRARY targets
Merge topic 'INTERFACE_LIBRARY-target-type' into next

7b9f660 export: Update approximation of version 2.8.13 to be over 2.8.12
Merge topic 'export-policy' into next

904ff9f export: Add policy CMP0024 to disallow include() of export files
Merge topic 'INTERFACE_LIBRARY-target-type' into next

ce0c303 install: Teach EXPORT option to handle INTERFACE_LIBRARY targets
435c912 export: Add support for INTERFACE_LIBRARY targets
fe73226 Add the INTERFACE_LIBRARY target type.