meson: compile compatibility sources separately

meson: compile compatibility sources separately

In the next commit we're about to introduce a precompiled header for
"git-compat-util.h". The consequence of this change is that we'll
implicitly include that header for every compilation unit that uses the
precompiled headers.

This is okay for our "normal" library sources and our builtins. But some
of our compatibility sources do not include the header on purpose, and
doing so would cause compilation errors.

Prepare for this change by splitting out compatibility sources into
their static library. Like this, we can selectively enable precompiled
headers for the library sources.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>