gnu: gnupg: Update to 2.1.19.

* gnu/packages/gnupg.scm (gnupg): Update to 2.1.19.
[arguments]: Add 'patch-test-paths' phase and remove 'set-gnupg-home' phase.
Add '--enable-gnupg-builddir-envvar' to #:configure-flags.
[source]: Add 'gnupg-2.1-fix-Y2038-test-failure.patch'.
* gnu/packages/patches/gnupg-2.1-fix-Y2038-test-failure.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.