meson: Make gzip and tar optional

They are only used for some tests.  The tests are already set to skip
as appropriate if they are not available.

Discussion: https://www.postgresql.org/message-id/flat/ZQzp_VMJcerM1Cs_%40paquier.xyz

contrib/basebackup_to_shell/meson.build

@@ -24,7 +24,7 @@ tests += {
     'tests': [
       't/001_basic.pl',
     ],
-    'env': {'GZIP_PROGRAM': gzip.path(),
-            'TAR': tar.path()},
+    'env': {'GZIP_PROGRAM': gzip.found() ? gzip.path() : '',
+            'TAR': tar.found() ? tar.path() : '' },
   },
 }