[PATCH] configure.ac: Avoid use of AC_DEFINE_DIR

AC_DEFINE_DIR is an old macro which doesn't seem to be available in
autoconf 2.72 on Debian 13, or in the autoconf-archive package. I think
it may not be necessary if we use AC_DEFINE_UNQUOTED instead here.

This change fixes a build error on Debian 13.
---
 configure.ac | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index ef8b5eb..78dc60b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -32,8 +32,7 @@ if test "$CXX" = "no"; then
     AC_MSG_ERROR([C++ compiler not found but is required])
 fi
 
-pkgdatadir=${datadir}/${PACKAGE}
-AC_DEFINE_DIR([PKGDATADIR], [pkgdatadir], [Where data are placed to.])
+AC_DEFINE_UNQUOTED([PKGDATADIR], ["$datadir/${PACKAGE}"], [Where data are placed to.])
 AC_SUBST(PACKAGE_NAME)
 
 dnl --with and --enable things
-- 
2.54.0