alloca considered harmful

[Matthias Andree]

Solaris:

/opt/SUNWspro/bin/cc -DBOGOFILTER -xtarget=ultra2 -xarch=v9 -g -xO3
-L/usr/local/BerkeleyDB.4.1//lib -R/usr/local/BerkeleyDB.4.1//lib  -o
bogofilter  bogofilter.o main.o method.o graham.o robinson.o fisher.o
libbogofilter.a  -lm  -ldb 
ild: (undefined symbol) alloca -- referenced in the text segment of
libbogofilter.a(lexer_v3.o)

Besides that, alloca is generally non-portable.

I'll try to back alloca changes back out if separate or try to find
proper free() calls if not.

-- 
Matthias Andree