Fixed a bug in the install target for include directories

This patch fixes two subtle problems with the `cs_install()` macro.

1.  The if statement would always fail. We tested this on both Ubuntu and OSX
2. If the files are installed to the package include destination, downstream packages will not be able to find the package headers because they will be in `install/include/package_name/`, rather than `install/include/`. The first option would require downstream packages to use a different include directory in headers depending on if they are compiling against the devel space or an overlaid install space.