Since revision 597 the command line for generating precompiled headers is broken.
The command line looks like this (when building a shared library in debug):
g++ -MMD -MP -Iobj/debug -include obj/debug/precompiled.hxx -g -fPIC -o "obj/debug/precompiled.hxx.gch" -MF obj/debug/precompiled.hxx.gch -c "precompiled.hxx"
The actual contents of obj/debug/precompiled.hxx.gch become the list of its dependencies. This leads to g++ mechanism for using precompiled headers to fail silently resulting in not actually using precompiled headers.