* gnu/packages/mingw.scm (mingw-w64): Update to 6.0.0. * gnu/packages/patches/mingw-w64-6.0.0-gcc.patch: New file. * gnu/packages/patches/mingw-w64-5.0rc2-gcc-4.9.3.patch: Delete it. * gnu/local.mk (dist_patch_DATA): Add new patch. Delete old patch. * gnu/build/cross-toolchain.scm (set-cross-path/mingw): Add additional autoconf-like substitutions.