Make linking APIs aware of 'head' target

The 'head' is the dependent target to be linked with the current target.
It will be used to evaluate generator expressions with proper handling
of mapped configurations and is used as the source target of properties.

This requires that memoization is done with a key of a pair of target
and config, instead of just config, because now the result also depends
on the target.  Removing the memoization entirely is not an option
because it slows cmake down considerably.
Add LINK_LIBRARIES property for direct target link dependencies

Previously we kept direct link dependencies in OriginalLinkLibraries.
The property exposes the information in the CMake language through the
get/set_property commands.  We preserve the OriginalLinkLibraries value
internally to support old APIs like that for CMP0003's OLD behavior, but
the property is now authoritative.  This follows up from commit d5cf644a
(Split link information processing into two steps, 2012-11-01).

This will be used later to populate the link interface properties when
exporting targets, and will later allow use of generator expressions
when linking to libraries with target_link_libraries.

Also make targets depend on the (config-specific) union of dependencies.
CMake now allows linking to dependencies or not depending on the config.
However, generated build systems are not all capable of processing
config-specific dependencies, so the targets depend on the union of
dependencies for all configs.
Add the target_include_directories command.

This is a convenience API to populate the corresponding properties.
Add the target_compile_definitions command.

This is a convenience API to populate the corresponding properties.
Merge topic 'target-includes-defines-commands' into next

4572e01 Add the target_compile_definitions command.
126cf5b Add the target_include_directories command.
Merge topic 'LINK_LIBRARIES-property' into next

7653862 Add LINK_LIBRARIES property for direct target link dependencies
40cf3fb Make linking APIs aware of 'head' target