Support for GNU thin archives when creating static libraries with qmake

When using the GNU toolchain, we create static libraries in a way that copies the object files into the archive.

GNU ar supports so-called thin archives that avoid copying the entire object file into the archive and instead stores the path to it in the archive, relative to the location of the .a file.

Thin archives are therefore much faster to create and result in overall less disk usage by avoiding the duplication of data.

It would be nice if qmake had built-in support for using thin archives if they are available. It should be opt-in, so that projects that use static libraries for internal usage (no intent of installing them) can use them if available.