build: Pass the right flags for "make distcheck".

* Makefile.am (AM_DISTCHECK_CONFIGURE_FLAGS): New variable.
* configure.ac: Define and substitute `NIX_PREFIX'.

Makefile.am

@@ -95,3 +95,5 @@ $(guix_install_go_files): install-nobase_dist_guilemoduleDATA
 SUBDIRS = po
 
 ACLOCAL_AMFLAGS = -I m4
+AM_DISTCHECK_CONFIGURE_FLAGS =					\
+  --with-nixpkgs=$(NIXPKGS) --with-nix-prefix="$(NIX_PREFIX)"

configure.ac

@@ -27,7 +27,11 @@ AC_ARG_WITH([nix-prefix],
   [AS_HELP_STRING([--with-nix-prefix=DIR], [search for Nix in DIR])],
   [case "$withval" in
     yes|no) ;;
-    *)      PATH="$withval/bin:$PATH"; export PATH;;
+    *)
+      NIX_PREFIX="$withval"
+      PATH="$NIX_PREFIX/bin:$PATH"; export PATH
+      AC_SUBST([NIX_PREFIX])
+      ;;
    esac],
   [])