Use top_srcdir/top_builddir in DOC_SOURCE_DIR
Use top_srcdir/top_builddir in DOC_SOURCE_DIR

to fix out-of-tree builds.

Bug #674443.

asivokon/vte-fx,thestinger/vte-ng,rvu95/vte,flaxandteal/gasket-vte,jessevdk/vte,rvu95/vte,rvu95/vte,saitoha/vte-sixel,pjm0616/vte,pjm0616/vte,flaxandteal/gasket-vte,saitoha/vte-sixel,saitoha/vte-sixel,rvu95/vte,jessevdk/vte,flaxandteal/gasket-vte,jessevdk/vte,flaxandteal/gasket-vte,thestinger/vte-ng,asivokon/vte-fx,thestinger/vte-ng,asivokon/vte-fx,pjm0616/vte,asivokon/vte-fx,jessevdk/vte