Since commit 2377ca28 (Removed the LastConfigureStep logic, 2012-07-26)
we exported targets one at a time and incrementally appended to
VTKTargets.cmake in the build tree.  This has two disadvantages:

(1) The export() command APPEND mode cannot detect target dependencies
that are not exported.

(2) If any target properties are changed after the export() command is
run then their build might not match the exported information.

We still collect all target names in a VTK_TARGETS global property.
Return to using it to export() all the targets at once.  Just call it
directly from the end of the top-level CMakeLists.txt file instead of
restoring the LastConfigureStep directory.

The VTK_INSTALL_EXPORT_NAME value is only for the install(EXPORT)
command, so return to hard-coding the name "VTKTargets.cmake" for the
build tree file.

Change-Id: If0fb56895a2fef34c4257ba607eb32b4f4a233ee