diff --git a/configure.ac b/configure.ac
index a82855a5..b7de03f7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -41,8 +41,8 @@ AC_SUBST(nix)
 old_CPPFLAGS="$CPPFLAGS"
 old_LIBS="$LIBS"
 
-CPPFLAGS="-I$nix/include/nix"
-LDFLAGS="-L$nix/lib/nix"
+CPPFLAGS="$CPPFLAGS -I$nix/include/nix"
+LDFLAGS="$LDFLAGS -L$nix/lib/nix"
 
 AC_CHECK_HEADER([store-api.hh], [:],
   [AC_MSG_ERROR([Nix headers not found; please install Nix or check the `--with-nix' option.])])