Sorry about that, I was travelling and didn't see that the buildbots
turned red. (I ran the test but didn't build python again)

I have changed that because build_ext.compiler is a string option, e.g.
the compiler type. Turning it into a compiler instance when the command
is run twice it breaks  because it is used as a string argument to
create a compiler.

Maybe a solution would be to rename the option name into "compiler_type"
and keep build_ext.compiler as a compiler instance.