gbuild, deps and ccache

Hi,

so the way gbuild generates dependencies prevents ccache to be confident about
using its cache most of the time. To see this: do a/ build b/ make clean
c/build from an empty ccache dir once with and without dep-tracking.

The solution IMHO is to make gbuild run g++ twice, if building with deps and
ccache:
  g++ -E $(DEP_GEN_OPTIONS) $(OTHER) foo.cxx &&
  ccache g++ $(OTHER) foo.cxx

The first will then only generate the deps without compiling, while the second
one will compile and not confuse ccache.

So: Are there volunteers for this to make the build quite a bit faster for
devs? If not I will have to EasyHackify this ...

Comments welcome.

Best,

Bjoern