meson: Increase minimum version to 0.57.2

The previous minimum was to maintain support for Python 3.5, but we
now require Python 3.6 anyway (commit 45363fca63), so that reason is
obsolete.  A small raise to Meson 0.57 allows getting rid of a fair
amount of version conditionals and silences some future-deprecated
warnings.

With the version bump, the following deprecation warnings appeared and
are fixed:

WARNING: Project targets '>=0.57' but uses feature deprecated since '0.55.0': ExternalProgram.path. use ExternalProgram.full_path() instead
WARNING: Project targets '>=0.57' but uses feature deprecated since '0.56.0': meson.build_root. use meson.project_build_root() or meson.global_build_root() instead.

It turns out that meson 0.57.0 and 0.57.1 are buggy for our use, so
the minimum is actually set to 0.57.2.  This is specific to this
version series; in the future we won't necessarily need to be this
precise.

Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/42e13eb0-862a-441e-8d84-4f0fd5f6def0%40eisentraut.org

src/test/modules/injection_points/meson.build

@@ -39,7 +39,7 @@ tests += {
       'reindex_conc',
       'vacuum',
     ],
-    'regress_args': ['--dlpath', meson.build_root() / 'src/test/regress'],
+    'regress_args': ['--dlpath', meson.project_build_root() / 'src/test/regress'],
     # The injection points are cluster-wide, so disable installcheck
     'runningcheck': false,
   },

src/test/modules/oauth_validator/meson.build

@@ -77,7 +77,7 @@ tests += {
       't/002_client.pl',
     ],
     'env': {
-      'PYTHON': python.path(),
+      'PYTHON': python.full_path(),
       'with_libcurl': oauth_flow_supported ? 'yes' : 'no',
       'with_python': 'yes',
     },

src/test/ssl/meson.build

@@ -7,7 +7,7 @@ tests += {
   'tap': {
     'env': {
       'with_ssl': ssl_library,
-      'OPENSSL': openssl.found() ? openssl.path() : '',
+      'OPENSSL': openssl.found() ? openssl.full_path() : '',
     },
     'tests': [
       't/001_ssltests.pl',