Fix crash on export of target with empty INTERFACE_INCLUDE_DIRECTORIES.

The new feature of install(TARGETS ... INCLUDES DESTINATION) introduced
in commit 650e61f8 (Add a convenient way to add the includes install
dir to the INTERFACE., 2013-01-05) introduced this crash. If the
new feature is used with a target which has no
INTERFACE_INCLUDE_DIRECTORIES, a segfault occurred.
Merge topic 'fix-export-includes-crash' into next

f868e47 Fix crash on export of target with empty INTERFACE_INCLUDE_DIRECTORIES.