For systems where gcc is used and where we don't know if GNU ld is

For systems where gcc is used and where we don't know if GNU ld is
used or not, let's ask collect2 which ld it uses and choose to use the
target do-gnu_shared if GNU ld is used.

This solves the reported problems on Solaris systems where GNU cc is
used but GNU ld isn't, and probably on other systems with similar
setups.