This will work iff AM_PROG_LEX uses AC_REQUIRE to invoke
AC_PROG_LEX.
Evidently so. From automake's m4/lex.m4:
AC_DEFUN([AM_PROG_LEX],
[AC_PREREQ([2.50])dnl
AC_REQUIRE([AM_MISSING_HAS_RUN])dnl
AC_REQUIRE([AC_PROG_LEX])dnl
if test "$LEX" = :; then
LEX=${am_missing_run}flex
fi])
