Force linking against the perl dll when using gcc 3.4 on Windows

When a Windows perl links an XS shared object to an import library using
gcc/g++ version 3.4.x, strange breakage happens at load time because the
mutex-related symbols seems not to be reachable. This situation happens
especially for AS perl and the default compiler suite it installs.

We fix this by forcing g++ to link against the dll instead, which should
be in the same directory as the perl executable (at least for AS perl).

This change is the integration of commit 14f8af2 from Variable::Magic's
repository.