Rename variable for including current directory in interfaces

Rename the variable added by commit 9ce1b9ef (Add
CMAKE_BUILD_INTERFACE_INCLUDES build-variable, 2012-11-25) to
CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE to be more consistent with the
existing CMAKE_INCLUDE_CURRENT_DIR variable.

Suggested-by: Alex Neundorf <neundorf@kde.org>
Merge topic 'rename-include-current-dir-in-interface-var' into next

fc43477 Rename variable for including current directory in interfaces