#4067 make distclean error

[Gabriele Gorla]

make distclean works but ends up with the following error:

....

+ DistCleaning root of the project...
make -f ./clean.mk distclean
make[1]: Entering directory '/mnt/scratch/build/sdcc/sdcc-svn/sdcc'
rm -f *core *[%~] *.[oa]
rm -f .[a-z]*~
make -C bin clean
make[2]: Entering directory '/mnt/scratch/build/sdcc/sdcc-svn/sdcc/bin'
make[2]: *** No rule to make target 'clean'.  Stop.
make[2]: Leaving directory '/mnt/scratch/build/sdcc/sdcc-svn/sdcc/bin'
make[1]: *** [clean.mk:6: clean] Error 2
make[1]: Leaving directory '/mnt/scratch/build/sdcc/sdcc-svn/sdcc'
make: *** [Makefile:239: distclean] Error 2

it seems the makefile in sdcc/bin is deleted before the script clean.mk runs.