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
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/master_win_compat'
Merge branch 'maint'