maintainer-makefile's _sc_search_regexp applies file name filters incorrectly when srcdir is not "."

_sc_search_regexp gives different results between VPATH configurations
and non-VPATH configurations.  By VPATH configurations, I mean build
trees that result from "../foo/configure" as opposed to "./configure".

AIUI, the problem is that the list of files offered to the filter is
different in each case.   For non-VPATH builds, vc-list-files is run
with the option "-C ." For VPATH builds it is run with the option "-C
../foo" or something like that.   The result is that the filtering
condition is applied to a stream of names which may or may not include
a $(srcdir)/ prefix.  Let's take sc_readme_link_copying as our
example.

In a non-VPATH build we get essentially this:

$  ./build-aux/vc-list-files -C . | grep -E '^README$'
README

For a VPATH build however, we get this:

$ ../cssc/build-aux/vc-list-files -C ../cssc | grep -E '^README$'

In other words, for maintainers who customarily use VPATH, some syntax
checks silently do nothing, only to fail later in "distcheck".
Changing "^" to "/" doesn't seem to be the right fix in the case of
sc_readme_link_copying because we only really want to perform this
check on the top-level README, and other files are likely to generate
false positives:

$ ../cssc/build-aux/vc-list-files -C ../cssc | grep -E '(^|/)README$'
../cssc/README
../cssc/tests/README
../cssc/testutils/README

The targets I suspect are affected by this problem include:

sc_changelog
sc_m4_quote_check (which will omit configure.ac)
sc_readme_link_install
sc_readme_link_copying

It looks to me as if there is an analogous problem with the in_files
filter, but  fortunately the checks that use it don't assume that ^X
will match a top-level file whose name starts with X.

VC_LIST_EXCEPT carefully removes the $(srcdir) prefix before filtering
with  exclude_file_name_regexp--<some suffix> and then re-adds the
prefix, but the same does not happen when applying the in_files and
in_vc_files filters.

Thanks,
James.