Apply some rlottie fixes.

Telegram/gyp/common.gypi

@@ -47,12 +47,14 @@
       'build_linux%': '<(build_linux)',
 
       'official_build_target%': '',
+      'build_standard_win%': 'c++17',
     },
     'build_os%': '<(build_os)',
     'build_win%': '<(build_win)',
     'build_mac%': '<(build_mac)',
     'build_linux%': '<(build_linux)',
     'official_build_target%': '<(official_build_target)',
+    'build_standard_win%': '<(build_standard_win)',
 
     # GYP does not support per-configuration libraries :(
     # So they will be emulated through additional link flags,

Telegram/gyp/lib_rlottie.gyp

@@ -8,6 +8,9 @@
   'includes': [
     'common.gypi',
   ],
+  'variables': {
+    'build_standard_win': 'c++14',
+  },
   'targets': [{
     'target_name': 'lib_rlottie',
     'type': 'static_library',
@@ -17,6 +20,7 @@
     ],
     'variables': {
       'official_build_target%': '',
+      'build_standard_win': 'c++14',
       'submodules_loc': '../ThirdParty',
       'libs_loc': '../../../Libraries',
       'rlottie_loc': '<(submodules_loc)/rlottie',

Telegram/gyp/settings_win.gypi

@@ -25,7 +25,7 @@
           'DebugInformationFormat': '3',          # Program Database (/Zi)
           'WarnAsError': 'true',
           'AdditionalOptions': [
-            '/std:c++17',
+            '/std:<(build_standard_win)',
             '/permissive-',
             '/Qspectre',
             '/MP',     # Enable multi process build.