coan: fix build on darwin

The configure script was failing because the `-std=c++11` flag was used
when trying to compile C.

Set the flag with CXXFLAGS instead.

pkgs/development/tools/analysis/coan/default.nix

···
 
   nativeBuildInputs = [ perl ];
 
-  NIX_CFLAGS_COMPILE = [
-    "-std=c++11"
-  ];
+  CXXFLAGS = "-std=c++11";
 
   enableParallelBuilding = true;
 
···
   '';
 
   meta = with lib; {
-    broken = stdenv.isDarwin;
     description = "The C preprocessor chainsaw";
     longDescription = ''
       A software engineering tool for analysing preprocessor-based