meson.build: switch to C++20

doc/developer.rst

@@ -12,7 +12,7 @@ Code Style
 * indent with tabs (width 8)
 * don't write CPP when you can write C++: use inline functions and constexpr instead of macros
 * comment your code, document your APIs
-* the code should be C++17 compliant, and must compile with :program:`GCC` 8 and :program:`clang` 5
+* the code should be C++20 compliant, and must compile with :program:`GCC` 10 and :program:`clang` 11
 * all code must be exception-safe
 * classes and functions names use CamelCase; variables are lower-case with words separated by underscore
 

doc/user.rst

@@ -55,7 +55,7 @@ and unpack it (or `clone the git repository
 
 In any case, you need:
 
-* a C++17 compiler (e.g. GCC 8 or clang 7)
+* a C++20 compiler (e.g. GCC 10 or clang 11)
 * `Meson 0.56.0 <http://mesonbuild.com/>`__ and `Ninja
   <https://ninja-build.org/>`__
 * Boost 1.58