Libraries from /usr/local/lib are not found when using gold linker

Hello,

on Linux `cpp.systemRunPaths` are auto detected via `ldconfig`. But the gold linker does not rely on values configured with/for ldconfig. So when trying to link against libraries which are placed in `/usr/local/lib` Qbs assumes that the linker searches automatically this directory too and does not append `-L/usr/local/lib`.

Either the `runPathsProbe` in `LinuxGCC.qbs` has to be updated to respect other linkers or `linkerFlags` in `gcc.js` should not remove `systemRunPaths`-paths from `libraryPaths`.