COMP: Use relative path for generate output header parameter

To work around CMake issue 12645:
http://www.cmake.org/Bug/view.php?id=12645

the generated header must be a relative path not an absolute. This has
been fixed in CMake 2.8.8, but the work around is easy enough not to
force a cmake upgrade.

Change-Id: Ie69235b8215e6056a14ddbe09e2e8bcd1942c8cb
Merge topic 'WorkAroundGenerateExportHeaderBug'

5d30673 COMP: Use relative path for generate output header parameter