Add custom build step to set up integrated build include directory.

This step copies all the headers that will be installed into the
install prefix at install time into the build area at build time.
This allows projects using an integrated OCE build to add a single
include path that captures all of the required header dependencies
to build without first running the install target.

Individual copies are made to work around teh 8192 character argument
limit with Visual Studio generators.
Add an OCE_COPY_HEADERS_BUILD option to enable this feature
Merge pull request #439 from camio/include-install-build-time

Add custom build step to set up integrated build include directory.