Fix configure bashisms with dash as /bin/sh as reported in
https://github.com/gentoo/gentoo/pull/27134

Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>

--- mailfilter-0.8.9/configure	2022-09-04 12:23:29.000000000 +0200
+++ mailfilter-0.8.9-dash/configure	2022-09-17 19:12:32.174505949 +0200
@@ -5322,7 +5322,7 @@ fi
 done
 test -n "$YACC" || YACC="yacc"
 
-if test x"$YACC" == x"yacc"
+if test x"$YACC" = x"yacc"
 then :
   as_fn_error $? "Please install bison or byacc before configuring." "$LINENO" 5
 fi
@@ -5545,7 +5545,7 @@ fi
 rm -f conftest.l $LEX_OUTPUT_ROOT.c
 
 fi
-if test x"$LEX" == x":"
+if test x"$LEX" = x":"
 then :
   as_fn_error $? "Please install flex before configuring." "$LINENO" 5
 fi