Pass -shared when linking some shared libraries

Pass -shared when linking some shared libraries

Currently, we use $(LT_LDLIB_shlib) to build most of our shared
libraries. This invokes libtool, passing our various flags like
PTH_LDFLAGS and PTH_CFLAGS (since all of our shared-library code is for
pthreads). Notably, we do NOT pass the -shared flag; the -shared flag
tells libtool to only build a shared library, and to not also build a
static library (on systems where libtool supports building shared and
static libraries simultaneously). Because of this, our LT_LDLIB_shlib
invocations build both, which is reasonably correct for our per-module
convenience libraries (that end up getting linked statically into the
binaries that we install), but is not entirely correct for the public
libraries that we install.  Specifically, for ABI compatibility
purposes, we must provide both shared and static libraries of the public
libraries that we install, and since libtool on AIX does not build (or
install) a static library at all with --mode-link unless -static is
passed, we have separate rules to build the shared and static libraries
for final installation.

This can cause install errors with parallel make (on non-AIX systems),
and possibly other errors, when we go to install the relevant library
into TOP_LIBDIR.  For example, in src/kopenafs, we have the following
rules:

    ${TOP_LIBDIR}/libkopenafs.${SHLIB_SUFFIX}: libkopenafs.la
            ${LT_INSTALL_DATA} libkopenafs.la ${TOP_LIBDIR}/libkopenafs.la
            ${RM} ${TOP_LIBDIR}/libkopenafs.la

    ${TOP_LIBDIR}/libkopenafs.a: libkopenafs.a
            ${INSTALL_DATA} libkopenafs.a $@

The rule to install libkopenafs.so will invoke libtool to do the
install, which will install libkopenafs.so, libkopenafs.so.X.Y, and
libkopenafs.a (from .libs/libkopenafs.a, not the libkopenafs.a we
built separately). If we are running the rule to install libkopenafs.a
in parallel, it may fail with an error like so:

    /usr/bin/install -c -m 644 libkopenafs.a /home/buildbot/openafs/fedora26-x86_64/build/lib/libkopenafs.a
    /usr/bin/install: cannot create regular file '/home/buildbot/openafs/fedora26-x86_64/build/lib/libkopenafs.a': File exists
    make[3]: *** [Makefile:35: /home/buildbot/openafs/fedora26-x86_64/build/lib/libkopenafs.a] Error 1

Even without that error, this confusion means that the libkopenafs.a
installed into TOP_LIBDIR may be the one from
src/kopenafs/libkopenafs.a, or the one from libtool's
src/kopenafs/.libs/libkopenafs.a; it depends on what order the rules
are run. If those libraries are different, that could potentially
cause all sorts of other problems.

To avoid this, we can pass -shared to libtool when building our shared
libraries. We used to pass -shared when building shared libraries,
since -shared is almost always one our SHLIB_LDFLAGS set in
src/osconf.m4. However, ever since commit 2c3a517e (Retire
Makefile.shared), SHD_CFLAGS, SHD_LDFLAGS, and SHD_CCRULE have all
been unused, and SHD_LDFLAGS was the only place where we used
SHLIB_LDFLAGS. As a result, we never use SHLIB_LDFLAGS anywhere, and
so we never pass -shared to anything.

However, we cannot pass -shared to libtool when building all of our
shared libraries, since we do need the static library for our per-module
convenience libraries. For example, liboafs_rx.la has no
separately-built static library (librx.a is for LWP, liboafs_rx.{so,a}
is for pthreads), but liboafs_rx needs to be linked statically into all
of our command-line tools.

So to fix this, introduce a new linking rule, called
LT_LDLIB_shlib_only, which causes the given library to be built only
as a shared library (by giving -shared to libtool), and not as a
static library. Update the build rules to use this new linking rule
for the libraries that need it, and leave the others alone. Since the
only use of LT_LDLIB_shlib_missing is also used for a public library
(afshcrypto), also pass -shared in that rule.

Also remove SHD_* and SHLIB_LDFLAGS variables, since they are unused.

Change-Id: Ia9e040afa3819f1ff70d050a400fecb9624bb9ba
Reviewed-on: https://gerrit.openafs.org/13786
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>