Cannot install built library to mingw sysroot

[Пётр Б.]

I have built and installed GraphicsMagick with this command:

$ ../configure '--host=x86_64-w64-mingw32' '--disable-shared'
--prefix=/usr/x86_64-w64-mingw32/sys-root/mingw CXXFLAGS="-flto
-ffat-lto-objects" && make && make install

Installation fails because apparently there is no permission to create
files in mingw sysroot:

$ mkdir /usr/x86_64-w64-mingw32/sys-root/mingw/share/whatever
mkdir: cannot create directory
‘/usr/x86_64-w64-mingw32/sys-root/mingw/share/whatever’: Permission
denied

What's the problem and how can I solve it in most straightforward way?

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