Move dependencies in 'external' directory and pdate git submodules

external/Catch2/meson.build

@@ -0,0 +1,19 @@
+#              Copyright Catch2 Authors
+# Distributed under the Boost Software License, Version 1.0.
+#   (See accompanying file LICENSE.txt or copy at
+#        https://www.boost.org/LICENSE_1_0.txt)
+
+# SPDX-License-Identifier: BSL-1.0
+
+project(
+  'catch2',
+  'cpp',
+  version: '3.5.4', # CML version placeholder, don't delete
+  license: 'BSL-1.0',
+  meson_version: '>=0.54.1',
+)
+
+subdir('src/catch2')
+if get_option('tests')
+  subdir('tests')
+endif