mxe configury: build native pkgconf and centralise various conf files

Since we need pkgconf for Cflags.private support, it seems like a
logical place for these types of config files instead of being
spread over Makefile, binutils, and gcc.
Merge pull request #319 from tonytheodore/mxe-configury

mxe configury: build native pkgconf and centralise various conf files