Reversing use_bzip2

[Mojca Miklavec]

Hi,

When trying to update x86_64-w64-mingw32-gcc I noticed that GCC 7.2 no
longer provides bzip2 sources, but I'm not sure how to tell the
Portfile that it should use xz instead of bzip2 given that the
crossgcc group already contains "use_bzip2 yes".

The alternative is of course to change all the Portfiles using the
crossgcc PortGroup to fetch from or to move "use_bzip2" to individual
Portfiles, but I'm sure there must be a better way?

Mojca