pkg_config arguments now come before ldflags when acquired from cgo_library

[peterebden]

I have a case where I need to add an extra flag for the output of pkg-config to work (I don't know why it comes off the system this way...). I should be able to do this via ldflags, however in the current setup those are added to the command line before the pkg-config invocation which is too early.

I think this seems more useful - I think it's not hyper-optimising just for this use case although it does feel quite specific.

[peterebden]

Okay I think a better way to solve my specific problem happens to be to invoke the compiler as clang++ instead of clang. I don't think this change is all bad but I'd rather not mess with it if I don't have to (I think there is scope for it breaking something else that relied on the current ordering).