[conan] share C++ standard declaration

CI/conan/base/android

@@ -1,8 +1,10 @@
+{% import 'vars.jinja' as vars %}
+
 include(common)
 
 [settings]
 compiler=clang
-compiler.cppstd=17
+compiler.cppstd={{ vars.cppstd }}
 compiler.libcxx=c++_shared
 compiler.version=14
 os=Android

CI/conan/base/apple

@@ -1,8 +1,10 @@
+{% import 'vars.jinja' as vars %}
+
 include(common)
 
 [settings]
 compiler=apple-clang
-compiler.cppstd=17
+compiler.cppstd={{ vars.cppstd }}
 compiler.libcxx=libc++
 compiler.version=16
 

CI/conan/base/msvc

@@ -1,8 +1,10 @@
+{% import 'vars.jinja' as vars %}
+
 include(common)
 
 [settings]
 compiler=msvc
-compiler.cppstd=17
+compiler.cppstd={{ vars.cppstd }}
 compiler.runtime=dynamic
 compiler.version=194
 os=Windows

CI/conan/base/vars.jinja

@@ -0,0 +1 @@
+{% set cppstd = '17' %}