The external command was being invoked with arguments: ["clang", ...,
"-isystem  /foo/bar/baz", ...], which meant that the compiler treated
the whitespace between the flag and the path as being part of the path
itself. (i.e. it was looking for "  /foo/bar/baz").

Fixed by passing the argument to execute_process() as
'-isystem/foo/bar/baz'; I also needed to ensure we normalized the value
of CMAKE_INCLUDE_SYSTEM_FLAG_<LANG>, which itself contains trailing
whitespace in some cases.

If the compiler understands system include paths it exposes the
necessary flag within the CMAKE_INCLUDE_SYSTEM_FLAG_${LANG}
variable.

System include paths for targets could be queried via the target
property INTERFACE_SYSTEM_INCLUDE_DIRECTORIES. Here the system
include paths are listed in addition of being available in
the INTERFACE_INCLUDE_DIRECTORIES property.

Note: Currently there is no way to query the SYSTEM flag for
directory include properties.

require CMake 2.8.6

* honor target property INCLUDE_DIRECTORIES

* CMake 2.8.8 compatibility

* Fix Xcode 4.3 compatibility.
* Cotire manual corrections.

* Cotire manual corrections.
* Add prefix header to the generated unity build target.

First release.