enabling msvc specific stuff in gcc

Just as note to myself (so I don't have to search all those ros-arm-bringup commits again to find it): the -fms-extensions gcc compiler switch: It enables such things like anonymous structs/unions:

typedef struct
{
    int a;
    int b;
} STRUCT1;

struct
{
    STRUCT1;
    int anotherone;
} x;