If "pv" is available at compile time, hard-code its path

configure.ac

@@ -171,6 +171,7 @@ NEED_PROG(bzip2, bzip2)
 AC_PATH_PROG(dot, dot)
 AC_PATH_PROG(dblatex, dblatex)
 AC_PATH_PROG(gzip, gzip)
+AC_PATH_PROG(pv, pv, pv)
 
 
 # Test that Perl has the open/fork feature (Perl 5.8.0 and beyond).

scripts/nix-copy-closure.in

@@ -63,7 +63,7 @@ while (@ARGV) {
         $includeOutputs = 1;
     }
     elsif ($arg eq "--show-progress") {
-        $progressViewer = "pv";
+        $progressViewer = "@pv@";
     }
     elsif ($arg eq "--dry-run") {
         $dryRun = 1;

substitute.mk

@@ -22,6 +22,7 @@
 	 -e "s^@sed\@^$(sed)^g" \
 	 -e "s^@tar\@^$(tar)^g" \
 	 -e "s^@gzip\@^$(gzip)^g" \
+	 -e "s^@pv\@^$(pv)^g" \
 	 -e "s^@tr\@^$(tr)^g" \
 	 -e "s^@dot\@^$(dot)^g" \
 	 -e "s^@xmllint\@^$(xmllint)^g" \