One export header directory
From the main commit message:
The VTK_MODULE_HEADER_DIR variable holds the path in the build tree to
use when generating headers for modules. The rationale is that the
number of include directories necessary when using a VTK build tree is
unweildly and can easily overflow a command line on Windows. While
response files work for compilers, not all tools or rules in CMake
support response files.
By using a single directory for generated headers, VTK can make it so
that only a limited number of modules need to add their binary directory
to the include paths.