meson: report detected runtime executable paths

Git needs to know about a couple of executable paths to pick at runtime.
This includes the system shell, but may also optionally include the Perl
and Python interpreters. Meson detects the location of these paths
automatically via `find_program()`, which does a lookup via the `PATH`
environment variable. As such, it may not be immediately obvious to the
developer which paths have been autodetected.

Improve this by exposing runtime executable paths at setup time.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

meson.build

@@ -2080,3 +2080,9 @@ summary({
   'sha256': sha256_backend,
   'zlib': zlib_backend,
 }, section: 'Backends')
+
+summary({
+  'perl': target_perl,
+  'python': target_python,
+  'shell': target_shell,
+}, section: 'Runtime executable paths')