[Bug modula2/108182] gm2 driver mishandles target and multilib options

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108182

--- Comment #12 from Iain Sandoe <iains at gcc dot gnu.org> ---

unfortunately, neither this nor the v4.1 (WIP) is still quite right.

Using LIBDIR in the computation of the include paths means that the compiler
does not work when it is relocated .. the directory prefix needs to be computed
at runtime.