[Pkg-gmagick-im-team] Magick++ ABI problem with GCC 7

[Jeroen Ooms]

I am unable to link my applications to ImageMagick++ when it is
compiled with gcc-7 due to an incompatibility in the `std::string()`
coercion of the `Magick::Color` class.

Below a minimal example:

    #include <Magick++.h>
    #include <iostream>

    int main(){
      Magick::Image image("logo:");
      Magick::Color col(image.fillColor());
      std::string strcol(col);
      std::cout << strcol;
      return 0;
    }

The program builds fine with g++6 but with g++7 I get a linking error:

    g++-6 test.cpp $(pkg-config --cflags --libs Magick++)
    ./a.out
    > #000000000000

However with gcc 7 we get:

    g++-7 test.cpp $(pkg-config --cflags --libs Magick++)
    /tmp/cchhBIuN.o: In function `main':
    test.cpp:(.text+0x7f): undefined reference to
`Magick::Color::operator std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> >() const'
    collect2: error: ld returned 1 exit status

Problem was also reported upstream but they say it's a compiler
problem: https://github.com/ImageMagick/ImageMagick/issues/707