add a thingy in meson to guess c++ std

since meson doesn't yet support cpp_std=c++2a or c++20 for msvc++,
only gcc (and the older version we support doesn't have c++20
either), default to none and add some minimal logic which will
guess it for whatever compiler we're using

people can still override the standard manually, in which case our
guess logic will not be used

README.md

@@ -110,7 +110,7 @@ compilers include:
 
 * GCC 10
 * Clang 11
-* Visual Studio 2019
+* Microsoft Visual C++ 2019
 
 Older versions of GCC and Clang may work, with no guarantees.
 

meson.build

@@ -1,8 +1,8 @@
 project('libcubescript', ['cpp'],
     version: '0.1.0',
     default_options: [
-        'buildtype=debugoptimized', 'cpp_std=c++2a',
-        'warning_level=3', 'cpp_rtti=false'
+        'buildtype=debugoptimized', 'warning_level=3', 'cpp_rtti=false',
+        'cpp_std=none'
     ],
     meson_version: '>=0.50'
 )
@@ -29,6 +29,24 @@ if get_option('buildtype') != 'plain'
     endif
 endif
 
+# Meson does not support C++20 std in a portable way in this version
+# unless specified explicitly, have our own logic which will guess it
+if get_option('cpp_std') == 'none'
+    if cxx.has_argument('-std=c++20')
+        # modern gcc/clang
+        extra_cxxflags += '-std=c++20'
+    elif cxx.has_argument('-std=c++2a')
+        # older gcc/clang
+        extra_cxxflags += '-std=c++2a'
+    elif cxx.has_argument('/std:c++20')
+        # future msvc++? not supported anywhere yet
+        extra_cxxflags += '/std:c++20'
+    elif cxx.has_argument('/std:c++latest')
+        # msvc++ 2019
+        extra_cxxflags += '/std:c++latest'
+    endif
+endif
+
 subdir('src')
 subdir('tools')