Add ./configure check for "lz4" command

Some environments may compile with --with-lz4 while the command "lz4"
goes missing, causing two failures in the TAP tests of pg_verifybackup
(008_untar.pl and 010_client_untar.pl) as the code assumed that the
command always existed with a hardcoded value in src/Makefile.global.
Rather than this method, this adds a ./configure check based on
PGAC_PATH_PROGS() to find automatically the command and get an absolute
path to it.

Both tests need to be adjusted for the case where the command does not
exist, actually, as Makefile.global would set now LZ4 to an empty value
in this case.  The TAP tests of pg_receivewal already do that.

Per report from buildfarm member copperhead, as an effect of dab2984.
The origin of the failure is actually babbbb5 that did not centralize
the check for the existence of a "lz4" command at ./configure to shave a
few cycles.  Note that one just needs to tweak an environment to move
"lz4" out of the way to reproduce the problem, which is what I did to
test this change.

Per discussion with Robert Haas, Tom Lane, Andres Freund and myself.

Discussion: https://postgr.es/m/Ygc51WVAFGocSu4h@paquier.xyz

configure

@@ -650,6 +650,7 @@ CFLAGS_ARMV8_CRC32C
 CFLAGS_SSE42
 have_win32_dbghelp
 LIBOBJS
+LZ4
 UUID_LIBS
 LDAP_LIBS_BE
 LDAP_LIBS_FE
@@ -13832,6 +13833,60 @@ fi
 
 fi
 
+if test -z "$LZ4"; then
+  for ac_prog in lz4
+do
+  # Extract the first word of "$ac_prog", so it can be a program name with args.
+set dummy $ac_prog; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_LZ4+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  case $LZ4 in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_LZ4="$LZ4" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_LZ4="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+  ;;
+esac
+fi
+LZ4=$ac_cv_path_LZ4
+if test -n "$LZ4"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $LZ4" >&5
+$as_echo "$LZ4" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+  test -n "$LZ4" && break
+done
+
+else
+  # Report the value of LZ4 in configure's output in all cases.
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for LZ4" >&5
+$as_echo_n "checking for LZ4... " >&6; }
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $LZ4" >&5
+$as_echo "$LZ4" >&6; }
+fi
+
 if test "$with_lz4" = yes; then
   for ac_header in lz4.h
 do :