--- configure.ac	2022-07-31 07:03:17.000000000 +0200
+++ configure.ac	2022-07-31 07:07:09.000000000 +0200
@@ -52,6 +52,10 @@
     [FLEXLEXER_DIR=$withval],
     [FLEXLEXER_DIR=""])
 
+if test -n "$FLEXLEXER_DIR"; then
+  CPPFLAGS="-I$FLEXLEXER_DIR $CPPFLAGS"
+fi
+ 
 LINK_GXX_STATICALLY=no
 AC_ARG_ENABLE(static-gxx,
     [AS_HELP_STRING([--enable-static-gxx],