Created attachment 553486 [details]
The problem seems to be:
configure: error: Pthreads are required to build libgomp
build logs are attached.
I have mingw winpthreads installed by re-emerging the libraries with USE=libraries like you suggested, but it seems this directory is not searched by the gcc build.
I have it here:
for me this was solved by reemerging mingw64-runtime with +libraries and then reemerging gcc
that's correct, if your cross-x86_64-w64-mingw32/gcc needs posix threads support, you first must build mingw64-runtime with USE="libraries"
*** This bug has been marked as a duplicate of bug 631460 ***