When building with --disable-introspection, we re-use the pre-generated
clients/common/settings-doc.h.in file.

When building with --enable-introspection, we generate
clients/common/settings-doc.h, and let `make check` verify that the
generated file is identical to what we would generate.

The common case where the generated file differ, is when code changed,
in this case, the developer is advised to update settings-doc.h.in.

Interpret environment variable NM_TEST_REGENERATE=1 to do this
automatically during `make check`.

This will be useful, as there might be several tests that compare a
generated file with a file from version control. NM_TEST_REGENERATE=1
will be the general way to re-generate all these files.

  $ NM_TEST_REGENERATE=1 make check