Can't build cscope without (f)lex

# make
No suffix list.
cd . && /bin/sh ./config.status config.h
config.status: creating config.h
config.status: config.h is unchanged
make all-recursive
No suffix list.
Making all in doc
No suffix list.
cd .. && /bin/sh ./config.status doc/Makefile
config.status: creating doc/Makefile
Making all in src
cd .. && /bin/sh ./config.status src/Makefile
depfiles
config.status: creating src/Makefile
config.status: executing depfiles commands
/bin/sh ../ylwrap `test -f 'fscanner.l' ||
echo './'`fscanner.l .c fscanner.c -- :
*** Error exit code 1

Stop.
*** Error exit code 1

Stop.
*** Error exit code 1

Stop.