Commit a14f0bf1 ("docs: convert build system documentation to rST")
converted docs/devel/build-system.txt to docs/devel/build-system.rst.

We still have several references to the old file, so let's fix them
with the following command:

  sed -i s/build-system.txt/build-system.rst/ \
      $(git grep -l docs/devel/build-system.txt)

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20210517151702.109066-4-sgarzare@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>