build: Only add -Wl,--as-needed when supported

build: Only add -Wl,--as-needed when supported

If -Wl,--as-needed is added to EXTRA_LDFLAGS (via ADD_LDFLAGS, as per
commit 996560191ac6bd603901dcd6c0de5d239e019ef4) then on some
platforms (at least CentOS 8 and Fedora 33), any indirect/recursive
dependencies (i.e. private libraries) are added to both the
binary (reqid_test in the CTDB case) and to samba-util.so.  However,
only samba-util.so has rpath set to find private libraries.

When ld.so tries to resolve these dependencies for the binary it
fails. This may be a bug on those platforms, but it occurs reliably
and our users will also hit the bug.  For binaries that have other
private library dependencies (e.g. bundled talloc) rpath will contain
the private library directory so the duplicate private library
dependencies are then found... that is, when it works, it works by
accident!

For some reason (deep in waf or wafsamba) if -Wl,--as-needed is added to
LINKFLAGS (as is done in conf.add_as_needed()) then it works: the direct
dependencies are only added to samba-util.so and the same depenencies
(indirect dependencies for binaries) are not added incorrectly to the
binaries.

So, without changing 1/2 of waf/wafsamba the simplest fix is to revert
to adding -Wl,--as-needed to LINKFLAGS, which was the case before
commit 996560191ac6bd603901dcd6c0de5d239e019ef4.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14288

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Bjoern Jacke <bj@sernet.de>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>