Hi,

When building gzip from the git repository in a subdirectory (VPATH build),
I get a build failure.

How to reproduce:

$ unset GNULIB_SRCDIR
$ git clone https://git.savannah.gnu.org/git/gzip.git
$ cd gzip
$ ./bootstrap
$ mkdir vpath
$ cd vpath
$ ../configure
$ make
$ make check
...
make[3]: Entering directory '/tmp/gzip/vpath'
  GEN      gzip.doc
  GEN      gzip.doc.gz
/bin/bash: line 1: ../gzip.doc: No such file or directory
make[3]: *** [Makefile:2435: gzip.doc.gz] Error 1
make[3]: Leaving directory '/tmp/gzip/vpath'

The problem is that while gzip.doc is expected to be in the source
directory (1. because it's in EXTRA_DIST, 2. see the check-local rule),
it's being generated in the build directory. The GNU Coding Standards
<https://www.gnu.org/prep/standards/html_node/Makefile-Basics.html>
make it clear that the file should be generated in the source directory.

The attached patch fixes it. I verified that "make distcheck" passes.