Fix compilation on MacOS / GHC 8.10.7 (#156)

Fixes errors along the lines of:
dear-imgui  > [ 2 of 17] Compiling DearImGui.GLFW
dear-imgui  > error: unknown type name 'constexpr'

So the "-std=c++11" option is not being passed to the C++ compiler on
MacOS.
The issue seems related to https://github.com/haskell/cabal/issues/6421

dear-imgui.cabal

@@ -185,6 +185,10 @@ library
     , vector
     , text
 
+  if os(darwin)
+    ghc-options:
+      -optcxx-std=c++11
+
   if flag(disable-obsolete)
     cxx-options: -DIMGUI_DISABLE_OBSOLETE_FUNCTIONS