From a clean source tree, if I build gitg with 'make -j1' then it builds fine.

If, however, if I build with 'make -j4' then the build typically fails with this:

make[2]: Entering directory `/home/adam/src/gitg/libgitg'
make[2]: *** No rule to make target `libgitg-1.0.vapi', needed by `all'.  Stop.
make[2]: *** Waiting for unfinished jobs....
  VALAC  libgitg_1_0_la_vala.stamp
make[2]: Leaving directory `/home/adam/src/gitg/libgitg'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/adam/src/gitg'
make: *** [all] Error 2

If I then type 'make -j4' again, it gets a bit further, then dies with a similar error:

make[2]: *** No rule to make target `libgitg-gtk-1.0.vapi', needed by `all'.  Stop.
make[2]: *** Waiting for unfinished jobs....
  VALAC  libgitg_gtk_1_0_la_vala.stamp
make[2]: Leaving directory `/home/adam/src/gitg/libgitg-gtk'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/adam/src/gitg'
make: *** [all] Error 2

Then on the third attempt the build completes.

So the gitg build is broken for parallel builds.  Presumably there's a dependency missing somewhere.