pgsql: Bring configure support for LZ4 up to snuff.

Bring configure support for LZ4 up to snuff.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4d399a6fbeb720b34d33441330910b7d853f703d

Modified Files
--------------
configure                  | 64 ++++++++++++++++++++++++++++++++++++++++++++--
configure.ac               | 18 +++++++++++--
src/include/pg_config.h.in | 11 +++++++-
src/tools/msvc/Solution.pm |  3 +++
4 files changed, 91 insertions(+), 5 deletions(-)