systemd: fix AM_PATH_LIBGCRYPT not found

This patch installs a copy of libgcrypt.m4 from the libgcrypt source tarball
to systemd m4 directory.

Libgcrypt uses a custom m4 macro and not pkg-config to check if the
development files are available. Though libgcrypt support is optional in
systemd, this macro should be available whenever autoreconf is used,
otherwise the re-configuration will fail with:

  configure.ac:616: warning: macro 'AM_PATH_LIBGCRYPT' not found in library

The call to autoreconf is required, as it is needed by the patch which
solves the `ln --relative` issue.

As asking the user to install the development package of libgcrypt on
the host machine or adding libgcrypt as a build dependency to systemd is
not acceptable, the required file is added to the m4 directory.

Fixes: http://autobuild.buildroot.net/results/1524d346fa17749e2ae62e063b9cfdd0de95c76a/
Fixes: http://autobuild.buildroot.net/results/10bcd92437eaa27eb61f8281c93efcb53d555e35/



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>