[Mingw-cross-env-list] qmake's pkg-config support needs to use mingw's version

[Nikos Chantziaras]

The mkspecs/features/link_pkgconfig.prf file needs to use 
'i686-pc-mingw32-pkg-config' instead of just 'pkg-config', otherwise the 
regular, non-mingw version is used:

# handle pkg-config files
for(PKGCONFIG_LIB, $$list($$unique(PKGCONFIG))) {
    QMAKE_CXXFLAGS += $$system(i686-pc-mingw32-pkg-config --cflags 
$$PKGCONFIG_LIB)
    QMAKE_CFLAGS += $$system(i686-pc-mingw32-pkg-config --cflags 
$$PKGCONFIG_LIB)
    LIBS += $$system(i686-pc-mingw32-pkg-config --libs $$PKGCONFIG_LIB)
}


This is needed for the following feature of qmake project files:

  CONFIG += link_pkgconfig
  PKGCONFIG += sdl SDL_mixer etc, etc