Do a dynamic link relative to the bin folder.

Instead of doing a global link to the position of the mono binaries use the relative position as they are both in the same folder.

Using a root based link is superfluous and creates issues with packaging systems not using fakeroot (like archlinux mono-git packages with makepkg), so this will generate a link mono -> mono-boehm instead of mono -> /usr/bin/mono-boehm (which would become the packaging folder in case of makepkg breaking the link)