binaries: Extend infrastructure for mingw

This includes some preparatory commits:

• Disable threading libraries where not needed, for gettext-runtime and libunistring.
• Only copy license files of enabled packages
• Specify license files for gettext

Then two commits that make it possible to cross-compile:

• Use LDFLAGS instead of LIBRARY_PATH
• Prepare Config for cross-compilation

And finally the extension for mingw.