Export: Test existing behavior of exporting includes with genexes.

Test that no warning or error diagnostic is issued for 'good code'.
Export: Process relative includes after genex evaluation.

In code such as

 install(TARGETS ...
   INCLUDES DESTINATION $<FOO>include
 )

the generator expressions are evaluated at generate-time. Delay
determining whether each entry is a relative path until after
the generator expressions are evaluated. Such relative paths
are based relative to the CMAKE_INSTALL_PREFIX.
Export: Process INSTALL_INTERFACE in INCLUDES DESTINATION.

Code such as

 install(TARGETS ...
   INCLUDES DESTINATION $<INSTALL_INTERFACE:include>
 )

should behave as if the INSTALL_INTERFACE wrapper were not present.
Export: Prefix relative items with genexes in INSTALL_INTERFACE.

Code such as

 target_include_directories(foo INTERFACE
   $<INSTALL_INTERFACE:include$<FOO>>
 )

should be treated as a relative directory, despite the genex, after
the INSTALL_INTERFACE is stripped away.

Previously, this would generate a relative directory on export, which
would be an error on import, so no policy is needed.
Export: Report error on relative include with genex.

Diagnostics which check the sanity of exported include paths
previously skipped over any path containing a generator expression.

Introduce a policy to issue an error message in such cases.

The export files created in the OLD behavior are not usable, because
they contain relative paths or paths to the source or build location
which are not suitable for use on installation. CMake will report an
error on import.
Merge topic 'export-includes' into next

5838aba Export: Report error on relative include with genex.
7a3e45b Export: Prefix relative items with genexes in INSTALL_INTERFACE.
f088a32 Export: Process INSTALL_INTERFACE in INCLUDES DESTINATION.
9eedc85 Export: Process relative includes after genex evaluation.
80790f3 Export: Test existing behavior of exporting includes with genexes.