AC_PROG_LEX doesn't notice when lex isn't installed

[Brian Sammon]

I notice that on a computer without some version of lex installed, the 
AC_PROG_LEX macro assumes that some version of lex is installed.  I would 
prefer that AC_PROG_LEX error out if lex is not installed.  I think it's safe 
to assume that packages that use AC_PROG_LEX require lex to build.

I'm willing to attempt to do the work myself it I knew that the patch would be 
accepted.

Similarly for AC_PROG_YACC

Any comments?