[support] Removing generated files

[Martti Kuparinen]

Hello UMIP developers,

I was having big problems to compile the latest UMIP code. All problems were in 
the end solved by manually running

cd src && rm -f gram.h gram.c scan.c

as "make distclean" did not remove those files.

Shouldn't clean and distclean remove those generated files? Could someone with 
write access to the repository please fix the Makefile.am...

Martti