make --with-pic default for autoconf

Add `--with-pic` to the default autoconf build.

This enables position independent code to be generated, which is necessary when using static libraries to build Foo.so (for Alien::Foo).  At the moment this is only useful if the packages does not build a dynamic library by default, but it may also be necessary if the static libs become the default (see #11).  I am creating this as a PR to open discussion on this specific part of #11.

Note that this conflicts with #46, and I will adjust this PR if both are accepted.