make clean on an incomplete build fails

[jdall]

If doing a 'make clean' on an incomplete build, the following error is displayed:

cd result && make -f makefile clean
make[1]: Entering directory '/media/sf_source/3rdparty/nativejson-benchmark/result'
rm *.html
rm: cannot remove '*.html': No such file or directory
makefile:13: recipe for target 'clean' failed
make[1]: *** [clean] Error 1
make[1]: Leaving directory '/media/sf_source/3rdparty/nativejson-benchmark/result'
Makefile:19: recipe for target 'clean' failed
make: *** [clean] Error 2

In the makefile in the 'result' folder, the following has to be changed:

clean:
-rm *.html
+rm -f *.html