http://bugs.gentoo.org/283029

--- a/m4/flex.m4
+++ b/m4/flex.m4
@@ -8,11 +8,11 @@ dnl ad_AC_PROG_FLEX
 dnl Look for flex or missing, then run AC_PROG_LEX and AC_DECL_YYTEXT
 AC_DEFUN(ad_AC_PROG_FLEX,
 [AC_CHECK_PROGS(LEX, flex, missing)
-if test "$LEX" = missing; then
+AS_IF([test "$LEX" = missing], [dnl
   LEX="\$(top_srcdir)/$ac_aux_dir/missing flex"
   LEX_OUTPUT_ROOT=lex.yy
   AC_SUBST(LEX_OUTPUT_ROOT)dnl
-else
+], [:
   AC_PROG_LEX
   AC_DECL_YYTEXT
-fi])
+])])