It's not okay to just shove the pkg_config results right into our
build flags, for a couple different reasons:
* This fails to maintain the separation between CPPFLAGS and CFLAGS,
as well as that between LDFLAGS and LIBS. (The CPPFLAGS angle is,
I believe, the reason for warning messages reported when building
with MacPorts' liblz4.)
* If pkg_config emits anything other than -I/-D/-L/-l switches,
it's highly unlikely that we want to absorb those. That'd be more
likely to break the build than do anything helpful. (Even the -D
case is questionable; but we're doing that for libxml2, so I kept it.)
Also, it's not okay to skip doing an AC_CHECK_LIB probe, as
evidenced by recent build failure on topminnow; that should
have been caught at configure time.
Model fixes for this on configure's libxml2 support.
It appears that somebody overlooked an autoheader run, too.
Discussion: https://postgr.es/m/20210119190720.GL8560@telsasoft.com
4d399a6fbe