install: Remove error condition using INCLUDES DESTINATION without EXPORT.

Commit 650e61f8 (Add a convenient way to add the includes install
dir to the INTERFACE., 2013-01-05) introduced an error case for
using the install(TARGETS) command with specified INCLUDES DESTINATION,
but no specified EXPORT set.

It is convenient to use a variable to set the various destinations
for different outputs (as KDE does), and some targets such as
executables are installed but not exported. This was triggering
the error case, but as it is a common case, remove the error.
Merge topic 'install-interface-includes' into next

72d13ff install: Remove error condition using INCLUDES DESTINATION without EXPORT.