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.ac

@@ -1485,6 +1485,7 @@ failure.  It is possible the compiler isn't looking in the proper directory.
 Use --without-zlib to disable zlib support.])])
 fi
 
+PGAC_PATH_PROGS(LZ4, lz4)
 if test "$with_lz4" = yes; then
   AC_CHECK_HEADERS(lz4.h, [], [AC_MSG_ERROR([lz4.h header file is required for LZ4])])
 fi