Add option to debug spirit parser
Required at least cmake 2.8.11

CMake function `target_compile_definitions` introduced since 2.8.11:

http://www.cmake.org/cmake/help/v2.8.11/cmake.html#command:target_compile_definitions
Merge pull request #10 from ruslo/debug.option

Provides a config-time switch to optionally produce Spirit-specific debug information in Debug builds