build: Set specific C++ standard to use (C++11)

There appears to be a difference in interpretation between clang and
gcc. Whereas gcc will by default pick the newest standard, clang picks
the oldest (or something like that). Enforce a specific standard.

It's good to be explicit but this is not really needed yet. It will be
when upgrading imgui, whose newer version uses constexpr and such.

meson.build

@@ -1,5 +1,5 @@
 project('sundog', 'c', 'cpp',
-        default_options: ['default_library=static', 'c_std=c99', 'warning_level=2'])
+        default_options: ['default_library=static', 'c_std=c99', 'cpp_std=c++11', 'warning_level=2'])
 
 sdl2_dep = dependency('SDL2')
 m_lib = meson.get_compiler('c').find_library('m', required: false)