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

configure

@@ -8699,8 +8699,18 @@ else
 $as_echo "yes" >&6; }
 
 fi
-  LIBS="$LZ4_LIBS $LIBS"
-  CFLAGS="$LZ4_CFLAGS $CFLAGS"
+  # We only care about -I, -D, and -L switches;
+  # note that -llz4 will be added by AC_CHECK_LIB below.
+  for pgac_option in $LZ4_CFLAGS; do
+    case $pgac_option in
+      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+    esac
+  done
+  for pgac_option in $LZ4_LIBS; do
+    case $pgac_option in
+      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+    esac
+  done
 fi
 
 #
@@ -12816,6 +12826,56 @@ fi
 
 fi
 
+if test "$with_lz4" = yes ; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for LZ4_compress_default in -llz4" >&5
+$as_echo_n "checking for LZ4_compress_default in -llz4... " >&6; }
+if ${ac_cv_lib_lz4_LZ4_compress_default+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_check_lib_save_LIBS=$LIBS
+LIBS="-llz4  $LIBS"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char LZ4_compress_default ();
+int
+main ()
+{
+return LZ4_compress_default ();
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  ac_cv_lib_lz4_LZ4_compress_default=yes
+else
+  ac_cv_lib_lz4_LZ4_compress_default=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+LIBS=$ac_check_lib_save_LIBS
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_lib_lz4_LZ4_compress_default" >&5
+$as_echo "$ac_cv_lib_lz4_LZ4_compress_default" >&6; }
+if test "x$ac_cv_lib_lz4_LZ4_compress_default" = xyes; then :
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_LIBLZ4 1
+_ACEOF
+
+  LIBS="-llz4 $LIBS"
+
+else
+  as_fn_error $? "library 'lz4' is required for LZ4 support" "$LINENO" 5
+fi
+
+fi
+
 # Note: We can test for libldap_r only after we know PTHREAD_LIBS
 if test "$with_ldap" = yes ; then
   _LIBS="$LIBS"