[tlbuild] TeX Live configure script should abort if lex cannot be found

[Ed Avis]

The configure script included in texlive-20080816-source checks for 
lex(1) but if not found it just prints 'checking for lex...no' and 
continues running.  This is a mistake, because if lex was not found 
then the variable LEX_OUTPUT_ROOT is unset, which causes 
texk/web2c/web2c/Makefile to be written with an incorrect filename:

LEX_OUTPUT_ROOT = @LEX_OUTPUT_ROOT@
[...]
web2c_objects = kps.o main.o y_tab.o $(LEX_OUTPUT_ROOT).o

If configure doesn't find lex then $(LEX_OUTPUT_ROOT) expands to an 
empty string which makes the build fail trying to make a file '.o' 
from a file '.c'.

Since lex appears to be required, I suggest making the configure 
script die if it's not found.

-- 
Ed Avis <ed at membled.com>