We run "git ls-files" in the build tree. Doesn't work when the source
directory isn't a git repository, or the build directory is outside
the source directory. Broken in commit 71cb2d8.
Find source files like Make.mk does: if the source tree is a git
repository, use git ls-files, else use sources.mk.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>
