Removing a gcc flag, --enable-version-specific-runtime-libs, that put gcc libs
in a speparate directory instead of /lib; this broke the installation of
libgcc_s.a for the case of "--enable-shared" in mingw-w64. And we already have all gccs in directories apart.
I also add the option --enable-fully-dynamic-string, which is used in the
prebuilt mingw64 toolchain; this way nixpkgs creates ABI-compatible binaries
with mingw64 upstream. (told by jon_y on irc ##mingw)
svn path=/nixpkgs/trunk/; revision=34242
44b2d21212