target_link_libraries: Update usage requirements documentation

Re-word the documentation to make clear that CMake integrates usage
requirements during generation and not synchronously during
configuration or execution of target_link_libraries.
Centralize maintenance of usage requirement include directories

Maintain a target's internal list of usage requirement include
directories whenever the LINK_LIBRARIES property is set by either
target_link_libraries or set_property.
Fix include dir propagation from conditionally linked targets

Generator expressions, including configuration-specific expressions may
be used as link libraries of targets.  The old-style keywords of
target_link_libraries are handled in terms of new generator expressions.
However, the generator expressions expect target names to be valid
against a regular expression, whereas target_link_libraries does not
require validation.  In generator expression constructed without any
action from the user we need to ensure that only valid expressions are
generated.  Ensure that strings which are not valid target names are not
used in generator expressions which validate the argument.

Code like

 target_link_libraries(B debug A)

generates usage requirement references such as "$<$<CONFIG:DEBUG>:A>".
When cmTarget::GetIncludeDirectories uses such references it generates
expressions like:

 $<TARGET_PROPERTY:$<$<CONFIG:DEBUG>:A>,INTERFACE_INCLUDE_DIRECTORIES>

When the conditions are false such references evaluate as an empty
string and the expression fails with an error such as:

 $<TARGET_PROPERTY:tgt,prop> expression requires a non-empty target name.

Fix this by teaching cmTarget::GetIncludeDirectories to wrap the above
expression inside a conditional:

 $<$<BOOL:$<$<CONFIG:DEBUG>:A>>:...>

so that $<TARGET_PROPERTY:...> will not be evaluated with an empty
target.
Memoize usage requirement include directories in a config-specific map

Commit 42ebb188 (Memoize includes and defines from interface libraries.,
2013-02-22) introduced caching of the includes.  Fix the memoization to
be configuration-specific so that we do not accumulate entries across
multiple evaluations in a multi-config generator.
Merge topic 'fix-per-config-tll-include-dirs'

dea1df4 Memoize usage requirement include directories in a config-specific map
26dba6a Fix include dir propagation from conditionally linked targets
b8259c3 Centralize maintenance of usage requirement include directories
Merge topic 'doc-tll-usage-requirements'

2618e02 target_link_libraries: Update usage requirements documentation