yywrap() undefined reference and 'flex --noyywrap' option

[Daniel J Sebald]

Below is an observation that perhaps someone with lots of Autotools 
experience can utilize to slightly clean up acinclude.m4.  Basically

../octave/configure LEX='flex --noyywrap'

seems to achieve the result of requiring 'flex' (as opposed to lex...is 
that what is wanted as HACKING says?) and determines no '-lfl' library 
is needed.

I tried making it work from inside configure.ac, but couldn't and 
decided to move on to programming.

Dan


configure:59019: gcc -o conftest -g -O2 -pthread   conftest.c  -lm   >&5
/tmp/cc4I4k2Q.o: In function `input':
/usr/local/src/octave/oct-build/lex.yy.c:1168: undefined reference to 
`yywrap'
/tmp/cc4I4k2Q.o: In function `yylex':
/usr/local/src/octave/oct-build/lex.yy.c:867: undefined reference to 
`yywrap'
/tmp/cc4I4k2Q.o: In function `main':
/usr/local/src/octave/oct-build/conftest.l:17: undefined reference to 
`yywrap'
collect2: ld returned 1 exit status
configure:59019: $? = 1
configure: failed program was:
| /* confdefs.h */

I read through the GNU Autotools documentation yesterday so I now 
understand the flex/yywrap error message.  The Autotools AC_PROG_LEX 
macro tries to figure out whether a library is needed.  The macro tries 
compiling with '', then '-lfl', then '-ll' and stops at whatever 
compiles correctly.  In this case, automake believes '-lfl' is required.

However, Octave supplies yywrap() so that the flex library isn't really 
needed.  Hence, in acinclude.m4 is the following override:

  AC_PROG_LEX
  case "$LEX" in
    flex*)
      LFLAGS="-I"
      AC_MSG_RESULT([defining LFLAGS to be $LFLAGS])
      LEXLIB=
    ;;
    *)

Autotools suggests the same in its documentation.  I.e.,
http://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Particular-Programs.html)

            AC_SUBST([LEXLIB], [''])

It seemed odd to me that Autotools would have such lengthy discussion 
about yywrap() and not have a clean way of dealing with it but instead 
suggest an after-the-fact fix.  Flex has an option of 'noyywrap' either 
at the command line or within the lexical scripts.  The option replaces 
yywrap with 1 (i.e., true).  For fun, I tried configuring with

../octave/configure LEX='flex --noyywrap'

which yields the desired result:

configure:58916: checking for flex
configure:58943: result: flex --noyywrap
configure:58980: flex --noyywrap conftest.l
configure:58984: $? = 0
configure:58986: checking lex output file root
configure:59000: result: lex.yy
configure:59005: checking lex library
configure:59019: gcc -o conftest -g -O2 -pthread   conftest.c  -lm   >&5
configure:59019: $? = 0
configure:59029: result: