doxygen: fixed documented include paths

* Docs say #include <gnuradio/blocks/add_ff.h> instead of <add_ff.h>, etc
wxgui: fix bug where grc_gnuradio files aren't added to NSIS installers
Merge remote-tracking branch 'ncorgan/wxgui_win32_cpack' into maint
Merge remote-tracking branch 'ncorgan/ncorgan/doxygen_path_fix' into maint
Windows compatibility fixes

* cmake: use CMake's internal variables for compiler info
* volk: void* can't be used for pointer arithmetic in MSVC, so cast
volk: use the explicit casting for all OSes, not just MSVC.