now testing WITH_APPS prior to add_subdirectory(apps)
conditionally include tests for apps
Clearing up some of the issues with pdal_defines.h

We have borrowed a number of helper macros from PCL and modify them for use in
PDAL. The pdal_defines.h should still be configured to the
PROJECT_BINARY_DIRECTORY, so we'll still need to test to see that it gets
updated accordingly.
Merge pull request #128 from chambbj/pdal_defines_rebase

compiles ok for me.

btw, I found a good solution for adding the git sha to the version string inside the utilities. I don't know how well it works on windows though. You can see it in action at http://github.com/hobu/hexer Can you give it a try and see if it works on windows for you there before I try to put it in PDAL?
Merge pull request #127 from chambbj/with_apps

Only build and test apps if the user has enabled WITH_APPS