maint(common): use cpp_std=c++17 in meson project default_options

We attempted to set the C++ version in standard.meson.build, but this
does not work well as the option name varies by platform, and it also
triggers a warning in meson. This fix moves the setting into each
meson.build project() default_options. As there are only a handful
of these, the maintenance burden is not high.

Fixes: #14432
Relates-to: #14355
Build-bot: build
Test-bot: skip

core/meson.build

@@ -9,6 +9,7 @@ project('keyman_core', 'cpp', 'c',
         version: files('../VERSION.md'),
         license: 'MIT',
         default_options : ['buildtype=release',
+                           'cpp_std=c++17',
                            'b_vscrt=static_from_buildtype',
                            'warning_level=2',
                            'debug=true'],

developer/src/kmcmplib/meson.build

@@ -8,7 +8,7 @@ project('kmcmplib', 'cpp', 'c',
         version: files('../../../VERSION.md'),
         license: 'MIT',
         default_options : ['buildtype=release',
-                           'cpp_std=c++14',
+                           'cpp_std=c++17',
                            'b_vscrt=static_from_buildtype',
                            'default_library=static',
                            'debug=true'])

linux/ibus-keyman/meson.build

@@ -1,5 +1,6 @@
 project('ibus-keyman', 'c', 'cpp',
         version: files('../../VERSION.md'),
+        default_options: ['cpp_std=c++17'],
         license: 'GPL-2+',
         meson_version: '>=1.0')
 

linux/keyman-system-service/meson.build

@@ -1,5 +1,6 @@
 project('keyman-system-service', 'c', 'cpp',
         version: files('../../VERSION.md'),
+        default_options: ['cpp_std=c++17'],
         license: 'MIT',
         meson_version: '>=1.0')
 

linux/mcompile/keymap/meson.build

@@ -1,6 +1,7 @@
 project(
   'mcompile', 'c', 'cpp',
   version: files('../../../VERSION.md'),
+  default_options: ['cpp_std=c++17'],
   license: 'MIT',
   meson_version: '>=1.0',
 )

mac/mcompile/meson.build

@@ -1,5 +1,6 @@
 project('mcompile', 'c', 'cpp',
         version: files('../../VERSION.md'),
+        default_options: ['cpp_std=c++17'],
         license: 'MIT',
         meson_version: '>=1.0')
 

resources/build/meson/standard.meson.build

@@ -16,12 +16,6 @@
 cpp_compiler = meson.get_compiler('cpp')
 c_compiler = meson.get_compiler('c')
 
-# Unfortunately this outputs a warning: 'Consider using the built-in option
-# for language standard version instead of using "-std=c++17".'
-# But that is not possible to do after creating the project...
-# See also https://github.com/mesonbuild/meson/issues/10685
-add_project_arguments('-std=c++17', language: 'cpp')
-
 #
 # Standard informational messages for our builds
 #