MinGW and packed struct alignment using C++11

Question

For the below structure, the actual (with no padding) size of the structure is 54. On a 64-bit (Windows 7) machine with MinGW (GCC) 4.8.1 x86_64, I get sizeof(BMPHeader) as 56, which is understandable. As per the requirement of the BMP file format, the structure should've no padding. I've three options (priority ordered):

1. C++11's alignas(1)
2. struct __attribute__ ((packed)) BMPHeader
3. #pragma pack(1)

However the last option (with least priority) alone seems to work giving me 54. Is this a bug in the compiler or I've completely mistook something here? The SSCCE

#include <iostream>

struct alignas(1) BMPHeader
{
    // BMP header
    uint16_t magic;
    uint32_t fileSize;
    uint32_t reserved;
    uint32_t dataOffset;

    // DIB header
    uint32_t dibHeaderLength;
    uint32_t width;
    uint32_t height;
    uint16_t numColourPlanes;
    uint16_t bitsPerPixels;
    uint32_t biBitFields;
    uint32_t dataSize;
    uint32_t physicalWidth;
    uint32_t physicalHeight;
    uint32_t numPaletteColours;
    uint32_t numImportantColours;
};

int main()
{
    std::cout << sizeof(BMPHeader) << std::endl;
}