Work around Meson bug ignoring c_std/cpp_std in subprojects

On Meson versions affected (or expected to be affected) by
mesonbuild/meson#14939, add C and C++ languages to the root project so
subprojects will respect c_std and cpp_std.

We can adjust the version range if needed once the bug is fixed.  There's
probably some value in continuing to carry this check for a while after
the fix is released, since developers might have older Meson versions on
their machines.

Author: bgilbert

meson.build

@@ -4,6 +4,19 @@ project(
   default_options: ['cpp_std=c++17'],
 )
 
+if (
+  meson.version().version_compare('<1.9.1') and
+  meson.version().version_compare('>=1.8.4')
+)
+  # ensure c_std= and cpp_std= are not ignored in subprojects
+  # https://github.com/mesonbuild/meson/issues/14939
+  add_languages(
+    'c',
+    'cpp',
+    native: false,
+  )
+endif
+
 wraps = get_option('wraps')
 
 foreach w : wraps