ltmain.in: Pass through all warning flags to the linker.

ltmain.in: Pass through all warning flags to the linker.

When link-time optimization (LTO) is used, some warnings are reported
while linking, and the flags must be passed to the linker to enable or
disable them.

-W* also catches -Wl,* and -Wa,*, but the former is already accounted
for in an earlier switch case, and the latter is already passed through.

* build-aux/ltmain.in: Pass through all warning flags to the linker.