cargo PG doesn't seem to work with the legacy-support PG

[RJVB (René Bertin)]

I may doing something wrong but it would seem that currently ports that are built via cargo fail when they need the legacy-support library/ies.

From what I can tell this is because rustc on Mac will invoke cc instead of ld for the final link step, and it does thus never get the legacy-support linker flags. Copying those flags from the ld compwrap into the cc compwrap solves the problem, letting the build run to completion.

Demonstration case: edit the sccache Portfile such that it runs cargo update (e.g. as its configure phase), set cargo.offline_cmd to an empty string and comment out the cargo.crates list. On 10.9 a port -nov build sccache will now fail in the final link step because of missing symbols provided by port:legacy-support (including linkat, renameat).